Das Geheimnis, um Profit in Paaren Trading zu finden. Quants ist der Name der Wall Street für Marktforscher, die quantitative Analysen verwenden, um profitable Handelsstrategien zu entwickeln. Kurz gesagt, ein Quant kämmt durch Preisverhältnisse und mathematische Beziehungen zwischen Unternehmen oder Handelsfahrzeugen, um profitable Handelsmöglichkeiten zu göttlichen In den 1980er Jahren eine Gruppe von Quants Arbeiten für Morgan Stanley schlug Gold mit einer Strategie namens der Paare Handel Institutionelle Investoren und proprietäre Handelstische bei großen Investmentbanken haben die Technik seitdem verwendet, und viele haben einen ordentlichen Gewinn mit der Strategie gemacht. Es ist selten im besten Interesse von Investmentbanker und Investmentfonds-Manager, um profitable Handelsstrategien mit der Öffentlichkeit zu teilen, so dass die Paare Handel blieb ein Geheimnis der Profis und ein paar geschickte Personen bis zum Aufkommen des Internets Online-Handel eröffnete den Deckel auf Echtzeit-Finanzinformationen und gab die Anfänger Zugang zu allen Arten von Anlagestrategien Es dauerte nicht lange für die Paare Handel, um einzelne Investoren und Kleinkind-Händler zu suchen, um ihre Risiko-Exposition gegenüber den Bewegungen des breiteren Marktes abzusichern. Was ist Paare Trading. Pairs Handel hat das Potenzial Um Gewinne durch einfache und relativ risikoarme Positionen zu erzielen Der Paarhandel ist marktneutral, was bedeutet, dass die Richtung des Gesamtmarktes seinen Gewinn oder Verlust nicht beeinflusst. Das Ziel ist es, zwei handelsfähige Fahrzeuge zu vergleichen, die in hohem Maße korreliert sind Die andere kurz, wenn das Paar-Preis-Verhältnis divergiert x Anzahl der Standardabweichungen - x wird mit historischen Daten optimiert Wenn das Paar auf seine mittlere Tendenz zurückkehrt, wird ein Gewinn an einer oder beiden der Positionen gemacht. Ein Beispiel mit Stocks. Traders können Verwenden Sie entweder grundlegende oder technische Daten, um ein Paar Trading-Stil zu konstruieren Unser Beispiel hier ist technischer Natur, aber einige Händler verwenden ein PE-Verhältnis oder andere grundlegende Faktoren, um Korrelation und Divergenz zu messen. Der erste Schritt bei der Gestaltung eines Paares Handel ist die Suche nach zwei Aktien, die Sind in hohem Maße korreliert Normalerweise bedeutet das, dass die Unternehmen in der gleichen Branche oder Subsektor sind, aber nicht immer. Zum Beispiel können Index-Tracking-Aktien wie die QQQQ Nasdaq 100 oder die SPY SP 500 hervorragende Paar-Trading-Chancen bieten Zwei Indizes, die im Allgemeinen zusammen handeln Sind die SP 500 und die Dow Jones Utilities Durchschnitt Diese einfache Preisplot der beiden Indizes zeigt ihre Korrelation. Für unser Beispiel werden wir auf zwei Unternehmen, die hoch korreliert GM und Ford sind, da beide amerikanischen Auto-Hersteller sind, ihre Aktien neigen dazu Bewegen sich zusammen. Below ist ein wöchentliches Diagramm des Preisverhältnisses zwischen Ford und GM berechnet durch Division von Ford s Aktienkurs von GM s Aktienkurs Dieses Preisverhältnis wird manchmal als relative Leistung nicht zu verwechseln mit der relativen Stärke Index etwas völlig anders Das Zentrum Weiße Linie stellt das mittlere Preisverhältnis in den vergangenen zwei Jahren dar. Die gelben und roten Linien stellen eine und zwei Standardabweichungen vom mittleren Verhältnis dar. In der nachstehenden Tabelle kann das Gewinnpotenzial identifiziert werden, wenn das Preisverhältnis auf das erste oder das Preisverhältnis trifft Zweite Abweichung Wenn diese gewinnbringenden Abweichungen auftreten, ist es Zeit, eine lange Position im Unterperformer zu nehmen und eine Short-Position im Overachiever Die Einnahmen aus dem Leerverkäufe können dazu beitragen, die Kosten der Long-Position zu decken, so dass die Paare Handel kostengünstig auf Position setzen Größe des Paares sollte durch den Dollar-Wert anstatt der Anzahl der Aktien auf diese Weise eine 5-Bewegung in einem gleich einer 5 bewegen in der anderen wie bei allen Investitionen, gibt es ein Risiko, dass die Trades könnte in die roten bewegen, so ist es Wichtig, um optimierte Stop-Loss-Punkte vor der Umsetzung der Paare Handel zu bestimmen. Ein Beispiel mit Futures-Kontrakten. Die Paar-Trading-Strategie funktioniert nicht nur mit Aktien, sondern auch mit Währungen, Rohstoffe und sogar Optionen Im Futures-Markt Mini-Verträge - kleinere Verträge, die Stellen einen Bruchteil des Wertes der Full-Size-Position dar - ermöglichen es kleineren Anlegern, mit Futures zu handeln. Ein Paarhandel im Futures-Markt könnte eine Arbitrage zwischen dem Futures-Kontrakt und der Cash-Position eines bestimmten Index beinhalten. Wenn der Futures-Kontrakt vorankommt Der Cash-Position, könnte ein Händler versuchen zu profitieren, indem sie die Zukunft zu kämpfen und gehen lange in der Index-Tracking-Aktien, erwarten, dass sie zusammen kommen an einem gewissen Punkt Oft sind die Bewegungen zwischen einem Index oder Rohstoff und seine Futures-Vertrag so eng, dass Gewinne sind Links nur für die schnellsten von Händlern - oft mit Computern automatisch ausführen enorme Positionen am Blinzeln eines Auges. Ein Beispiel mit Optionen. Option Trader verwenden Anrufe und setzt auf Risiken abzusichern und nutzen Flüchtigkeit oder das Fehlen davon Ein Anruf ist eine Verpflichtung durch Der Schriftsteller, um Aktien einer Aktie zu einem bestimmten Preis irgendwann in der Zukunft zu verkaufen Ein Put ist eine Verpflichtung des Schriftstellers, Aktien zu einem bestimmten Preis irgendwann in der Zukunft zu kaufen Ein Paar Handel auf dem Optionsmarkt könnte beinhalten, einen Aufruf für eine Sicherheit zu schreiben Das übertrifft sein Paar eine andere hochkorrelierte Sicherheit und passt die Position an, indem sie ein Put für das Paar die Underperforming-Sicherheit schreibt. Da die beiden zugrunde liegenden Positionen wieder auf ihren Mittelwert zurückgehen, werden die Optionen wertlos, so dass der Trader den Erlös von einem oder beiden ablegen kann Der Positionen. Evidence of Profitability Im Juni 1998 veröffentlichte Yale School of Management ein Papier von Even G Gatev, William Goetzmann und K Geert Rouwenhorst, die versucht zu beweisen, dass Paare Handel ist profitabel Mit Daten von 1967 bis 1997, die Trio fand, dass über einen halbmonatigen Handelsperiode, die Paare Handel durchschnittlich eine 12 Rückkehr Um profitable Ergebnisse von einfachem Glück zu unterscheiden, enthielt ihre Prüfung konservative Schätzungen der Transaktionskosten und zufällig ausgewählte Paare Sie können das vollständige 34-seitige Dokument hier finden Interessiert an den Paaren Handelstechnik finden Sie weitere Informationen und Unterricht in Ganapathy Vidyamurthy s Buch Pairs Trading Quantitative Methoden und Analysen, die Sie hier finden können. Der breite Markt ist voller Höhen und Tiefen, die schwache Spieler zu erzwingen und selbst die klügsten Prognostikern glücklicherweise verwechseln , Mit marktneutralen Strategien wie dem Paar Handel, Investoren und Händler können Gewinne in allen Marktbedingungen finden Die Schönheit der Paare Handel ist seine Einfachheit Die lange kurze Beziehung von zwei korrelierten Wertpapieren fungiert als Ballast für ein Portfolio in den abgehackten Gewässern gefangen Des Gesamtmarktes Viel Glück mit Ihrer Jagd nach Profit in Paaren Handel, und hier s zu Ihrem Erfolg in den Märkten. Die maximale Höhe der Gelder der Vereinigten Staaten können leihen Die Schulden Decke wurde unter dem Zweiten Liberty Bond Act erstellt. Der Zinssatz Bei dem ein Depotinstitut die Geldwährung an der Federal Reserve an eine andere Depotbank leiht.1 Eine statistische Maßnahme für die Streuung der Renditen für einen bestimmten Wertpapier oder Marktindex Die Volatilität kann entweder gemessen werden. Handeln Sie den US-Kongress, der 1933 als Banking verabschiedet wurde Act, die Geschäftsbanken von der Teilnahme an der Investition verboten. Nonfarm Lohn-und Gehaltsliste bezieht sich auf jede Arbeit außerhalb der landwirtschaftlichen Betriebe, private Haushalte und der gemeinnützige Sektor Das US-Büro der Arbeit. Die Währung Abkürzung oder Währungssymbol für die indische Rupie INR, die Währung von Indien Die Rupie besteht aus 1. Ein Verfahren ist vorgesehen, um eine Paarhandelsanforderung zu erfüllen, und umfasst die Schritte des Empfangens einer Vielzahl von Paarhandelsanforderungen, die eine Transaktion für einen ersten Teil einer der Vielzahl von Paarhandelsanforderungen ausführen und eine Sekunde anpassen Teil der einen der Vielzahl von Paarhandelsanforderungen gegen einen anderen der Vielzahl von Paarhandelsanforderungen. 46.1 Computerimplementiertes Verfahren zur Erfüllung einer Paarhandelsanforderung mit den Schritten des Erhaltens einer Vielzahl von Paarhandelsanforderungen, die eine Paarhandelsanforderung und eine andere Paarhandelsanforderung umfasst, wobei jede Paarhandelsanforderung eine Anforderung umfasst, eine erste Sicherheit zu handeln, Eine Anforderung, eine zweite Sicherheit zu handeln, und eine Anforderung, die erste Sicherheit und die zweite Sicherheit mit einer minimalen Spreizgrenze zu tauschen, und wobei die erste Sicherheit und die zweite Sicherheit jeweils einen Gebotspreis und einen Ask-Preis haben. Bestimmen der Gebotsausgabe Auf dem Markt der ersten Sicherheit und der zweiten Sicherheit. Bestimmen der fragen fragen verbreitet in den Markt der ersten Sicherheit und die zweite Sicherheit. destimmt, dass die minimale Spreizgrenze jeder Paarhandelsanforderung durch eine Reihe der Gebotsgebotsausbreitung erfüllt wird Und fragt fragen fragen, dass eine Transaktion zwischen einem ersten Teil des Handels der ersten Sicherheit in der einen Paar-Handelsanforderung und mindestens einer Nicht-Paar-Handelsanforderung ausgeführt wird, vorausgesetzt, dass die minimale Spreizgrenze der einen Paarhandelsanforderung erfüllt ist Durch den Bereich des Gebotsgebots verbreitet und die Frage fragen verbreiten und. Mischen, unter Verwendung eines Computers, einen zweiten Teil des Handels der ersten Sicherheit in der einen Paarhandelsanforderung und mindestens einen ersten Teil des Handels der zweiten Sicherheit In der einen Paarhandelsanforderung gegen die andere Paarhandelsanforderung, vorausgesetzt, daß ein Bereich der minimalen Spreizgrenze der einen Paarhandelsanforderung und der anderen Paarhandelsanforderung mit dem Bereich der Gebotsausbreitungsspanne überlappt und die Frage fragen, Verfahren nach Anspruch 1, wobei der Schritt des Ausführens einer Transaktion für den ersten Teil des Handels des ersten Wertpapiers in der einen Paar-Handelsanforderung den Schritt des Ausführens einer Transaktion für den ersten Teil des Handels des ersten Wertes in dem Ein Paar Handelsanforderung in einem externen Markt.3 Verfahren nach Anspruch 1, wobei der Schritt des Ausführens einer Transaktion von einem Finanzinstitut durchgeführt wird, das einen Auftragsbestand hat und der Schritt des Ausführens einer Transaktion den Schritt des Ausführens einer Transaktion für die erste umfasst Teil des Handels des ersten Wertpapiers in der einen Paarhandelsanforderung gegen den Auftragsinventar.4 Verfahren nach Anspruch 1, ferner umfassend :Verwenden einer Transaktion zwischen einem zweiten Teil des Handels des zweiten Wertpapiers in der einen Paarhandelsanforderung und an Mindestens eine andere Nicht-Paar-Handelsanforderung, vorausgesetzt, dass die minimale Spreizgrenze der Ein-Paar-Handelsanforderung durch den Bereich der Gebotsausbreitungsspanne und der fraglichen Sprechspanne erfüllt ist und wobei der Schritt des Ausführens des ersten Teils des einen Paarhandels erfolgt Anfordern und Ausführen des zweiten Teils der einen Paarhandelsanforderung umfasst die Schritte: Bestimmen, ob der Geldkurs der ersten Sicherheit und der Geldkurs der zweiten Sicherheit eine Spreizgrenze erfüllt. Bestimmen eines Betrags der zweiten Sicherheit, die verkauft werden kann Basierend auf einer Gebotsgröße, die mit der zweiten Sicherheit assoziiert ist, Berechnen einer äquivalenten Menge des ersten Wertpapiers, die auf der Grundlage der Menge der zweiten Sicherheit, die verkauft werden kann, gekauft werden kann. Einstellen der äquivalenten Menge der ersten Sicherheit auf der Grundlage von Anpassungskriterien. Berechnen Einen Kaufpreis für den bereinigten äquivalenten Betrag des ersten Wertpapiers auf der Grundlage der Spread-Limite. Eine Ausführung eines Einleitungsauftrags, um den angepassten äquivalenten Betrag des ersten Wertpapiers zu dem Kaufpreis zu kaufen, undAusführen eines Deckungsauftrags, um den Betrag der zweiten Sicherheit zu verkaufen .5 Verfahren nach Anspruch 4, wobei der Schritt des Ausführens eines Deckungsauftrags zum Verkauf den Schritt des Ausführens eines Deckungsauftrags umfasst, um den Betrag der zweiten Sicherheit zum Kaufpreis der zweiten Sicherheit zu verkaufen.6 Das Verfahren nach Anspruch 4 weiter Mit den Schritten des Bestimmens, ob der Ask-Preis der ersten Sicherheit und der Ask-Preis der zweiten Sicherheit eine Spreizgrenze erfüllt. Bestimmen eines Betrags der ersten Sicherheit, die auf der Grundlage einer Angebotsgröße gekauft werden kann, die mit der ersten Sicherheit verbunden ist Eine äquivalente Menge des zweiten Wertpapiers, die auf der Grundlage des Betrags der zweiten Sicherheit verkauft werden kann, die gekauft werden kann. Einrichten des äquivalenten Betrags der zweiten Sicherheit auf der Grundlage von Anpassungskriterien. Berechnen eines Verkaufspreises für den angepassten äquivalenten Betrag der zweiten Sicherheit Basierend auf der Spread-Begrenzung. Entwicklung eines Einleitungsauftrags, um den angepassten äquivalenten Betrag des zweiten Wertpapiers zu dem Verkaufspreis zu verkaufen, undVerwenden eines Deckungsauftrags zum Erwerb des Betrags der ersten Sicherheit.7 Verfahren nach Anspruch 6, wobei der Schritt des Ausführens Ein Deckungsauftrag zum Erwerb der Schritt des Ausführens eines Deckungsauftrags zum Erwerb des Betrags der ersten Sicherheit zum Kaufpreis der ersten Sicherheit umfasst. Verfahren nach Anspruch 6, wobei die Anpassungskriterien einen Mindestbetrag und einen Höchstbetrag umfassen. Verfahren nach Anspruch 8, wobei der Schritt des Ausführens einer Einleitungsreihenfolge den Schritt des Umrichtens der Einleitungsreihenfolge auf eine runde Losgröße umfasst.10 Verfahren nach Anspruch 1, wobei der Schritt des Ausführens eines ersten Teils des Handels des ersten Wertpapiers umfasst In der einen Paar-Handelsanforderung den Schritt des Ausführens eines ersten Teils des Handels des ersten Wertpapiers in der einen Paarhandelsanforderung in einer Vielzahl von Tranchen enthält.11 Verfahren nach Anspruch 1, wobei die eine Paarhandelsanforderung eine erste Spreizung aufweist Begrenzen und die andere Paarhandelsanforderung eine zweite Spreizgrenze hat und wobei der Schritt des Anpassens des zweiten Teils des Handels der ersten Sicherheit in der einen Paarhandelsanforderung und wenigstens ein erster Teil des Handels der zweiten Sicherheit in dem einen ist Paar-Handelsanforderung gegen die andere Paarhandelsanforderung enthält ferner die Schritte des Bestimmens, dass ein Bereich der ersten Spreizgrenze und die zweite Spreizgrenze mit einer Marktspreizung überlappt. Sprechen eines Spread Levels. Berechnen der Preise für die erste Sicherheit und die zweite Sicherheit Die sich innerhalb des Marktes befinden und auf der Grundlage des gespreizten Ausmaßes basieren und den zweiten Teil des Handels der ersten Sicherheit in der einen Paarhandelsanforderung und mindestens einen ersten Teil des Handels der zweiten Sicherheit in der einen Paarhandelsanforderung erfassen Gegen die andere Paarhandelsanforderung auf der Grundlage der berechneten Preise.12 Verfahren nach Anspruch 11, wobei der Schritt des Setzens eines Spreizpegels die Schritte umfasst: Berechnen eines Mittelwerts zwischen der ersten Spreizgrenze und der zweiten Spreizgrenze undVerfahren des Spreizpegels Wie es heißt, wenn das Mittel innerhalb der Marktspreizung liegt.13 Verfahren nach Anspruch 12, ferner mit dem Schritt des Identifizierens eines Spreizbetrags, der dem Mittelwert und dem Marktaufteil am nächsten liegt, undVerwenden des Spreizniveaus als Spreizungsbetrag, wenn Der Mittelwert nach Anspruch 1, wobei die Ein-Paar-Handelsanforderung eine erste Spreizgrenze, eine Kaufquote und ein Verkaufsverhältnis aufweist, sagte die andere Paarhandelsanforderung eine zweite Spreizgrenze, ein Kaufverhältnis und Ein Verkaufsverhältnis und wobei der Schritt des Anpassens eines zweiten Teils des Handels der ersten Sicherheit in der einen Paarhandelsanforderung und wenigstens der erste Teil des Handels der zweiten Sicherheit in der einen Paarhandelsanforderung gegen die andere Paarhandelsanforderung erfolgt Ferner die folgenden Schritte umfasst: Bestimmen, dass das Kaufverhältnis und das Verkaufsverhältnis, das mit der einen Handelsanforderung verbunden ist, nicht gleich dem Kaufverhältnis und dem Verkaufsverhältnis der anderen Paarhandelsanforderung entspricht und dass eine Überlappung zwischen dem Bereich der ersten Spreizgrenze besteht Und die zweite Spreadgrenze und eine Marktspreizung. Bestimmen, dass die Marktpreise existieren, die innerhalb der Überlappung liegen. Bestimmen eines Fehlanpassungsbetrags in der zweiten Sicherheit auf der Grundlage eines Unterschieds zwischen dem Kaufverhältnis und dem Verkaufsverhältnis, das mit der einen Paarhandelsanforderung verbunden ist, und sagte Kauf-Verhältnis und das Verkaufsverhältnis der anderen Paarhandelsanforderung. Berechnen eines Kreuzbetrages für die erste Sicherheit und die zweite Sicherheit. Wählen eines Kreuzungspreises für die erste Sicherheit und die zweite Sicherheit, die innerhalb der Überlappung liegt. Bestimmen, dass die Fehlanpassungsmenge ist Die bei dem Kreuzungspreis für die zweite Sicherheit verfügbar ist. Erfahren des zweiten Teils des Handels der ersten Sicherheit in der einen Paarhandelsanforderung und wenigstens der erste Teil des Handels der zweiten Sicherheit in der einen Paarhandelsanforderung gegen das andere Paar Handelsanforderung auf der Grundlage der ausgewählten Preise undVerwenden einer Transaktion für den Nichtübereinstimmungsbetrag der zweiten Sicherheit zu dem Kreuzungspreis für die zweite Sicherheit.15 Verfahren nach Anspruch 14, wobei der Schritt des Bestimmens, dass die Fehlanpassungsmenge bei dem Kreuzungspreis verfügbar ist Für die zweite Sicherheit den Schritt des Bestimmens, dass der Fehlanpassungsbetrag in einem externen Markt zu dem Kreuzungspreis für die zweite Sicherheit verfügbar ist.16 Verfahren nach Anspruch 14, wobei der Schritt des Bestimmens, dass die Fehlanpassungsmenge verfügbar ist, durch a durchgeführt wird Ein Finanzinstitut mit einem Auftragsbestand und der Schritt des Bestimmens, dass der Fehlanpassungsbetrag bei dem Kreuzungspreis für die zweite Sicherheit verfügbar ist, umfasst den Schritt des Bestimmens, dass die Fehlanpassungsmenge in dem besagten Bestandsverzeichnis bei dem Kreuzungspreis für die zweite Sicherheit verfügbar ist. Verfahren nach Anspruch 1, wobei die eine Paarhandelsanforderung und die andere Paarhandelsanforderung eine Anzahl von Spreads angeben, und wobei der Schritt des Anpassens eines zweiten Teils des Handels der ersten Sicherheit in der einen Paarhandelsanforderung und mindestens der ersten Teil des Handels der zweiten Sicherheit in der einen Paarhandelsanforderung gegen die andere Paarhandelsanforderung umfasst den Schritt des Erfassens eines zweiten Teils des Handels des ersten Wertes in der einen Paarhandelsanforderung und mindestens den ersten Teil von Der Handel der zweiten Sicherheit in der einen Paarhandelsanforderung gegen die andere Paarhandelsanforderung, wenn die Anzahl der Spreads größer als eine minimale Anzahl von Spreads ist.18 Verfahren nach Anspruch 1 mit dem Schritt des Erhaltens einer Präferenz zum Füllen zumindest Wobei einige der Mehrzahl von Handelsanforderungen über den Ausführungsschritt vorliegen.19 Verfahren nach Anspruch 1 mit dem Schritt des Erhaltens einer Präferenz zum Füllen von zumindest einigen der Vielzahl von Handelsanforderungen über den Anpassungsschritt.20 Verfahren nach Anspruch 1, wobei a Client eine Abfrage bezüglich eines Status der Paarhandelsanforderung einreicht und der Status der Paarhandelsanforderung kontinuierlich in Echtzeit aktualisiert wird.21 Verfahren nach Anspruch 19, wobei die Präferenz von einem Kunden elektronisch vorgelegt wird.22 Verfahren nach Anspruch 1, wobei: Ein Klient einen gleichzeitigen Bericht erhält, wenn die Paarhandelsanforderung gefüllt ist und der Klient das Paar Handel elektronisch bestätigt.23 Verfahren nach Anspruch 1, ferner umfassend: Ausüben einer Transaktion zwischen einem zweiten Teil des Handels des zweiten Wertpapiers in dem einen Paar Handel Anforderung und mindestens eine andere Nicht-Paar-Handelsanforderung, vorausgesetzt, dass die minimale Spreizgrenze der einen Paarhandelsanforderung durch den Bereich der Gebotsausbreitungsspanne und der fraglichen Spread gespreizt wird, und wobei der Schritt des Ausführens des ersten Teils des Eine Paarhandelsanforderung und das Ausführen des zweiten Teils der einen Paarhandelsanforderung umfasst den Schritt des Bestimmens, ob der fragenpreis der ersten Sicherheit und der fragenpreis der zweiten sicherheit eine Spreizgrenze erfüllt. Bestimmen eines Betrags der zweiten Sicherheit, die Kann auf der Grundlage einer mit der zweiten Sicherheit verbundenen fragengröße gekauft werden, die einen äquivalenten Betrag des ersten Wertpapiers berechnet, der auf der Grundlage des Betrags der zweiten Sicherheit verkauft werden kann, die gekauft werden kann. Einstellen der äquivalenten Menge der ersten Sicherheit auf der Grundlage der Anpassung Kriterien, die einen Verkaufspreis für den bereinigten äquivalenten Betrag des ersten Wertpapiers auf der Grundlage der Spreizgrenze berechnen. Verwendung eines Einleitungsauftrags, um den angepassten äquivalenten Betrag des ersten Wertpapiers zu dem Verkaufspreis zu verkaufen, undVerwenden eines Deckungsauftrags, um diesen Betrag zu erwerben Die zweite Sicherheit.24 Ein System zur Erfüllung einer Paarhandelsanforderung, wobei das System eine Vielzahl von Paarhandelsanforderungen empfängt, die eine Paarhandelsanforderung und eine andere Paarhandelsanforderung umfasst, wobei jede Paarhandelsanforderung eine Anforderung zum Handel einer ersten Sicherheit umfasst, a Um eine zweite Sicherheit zu handeln, und eine Anforderung, die erste Sicherheit und die zweite Sicherheit mit einer minimalen Spreizgrenze zu tauschen, und wobei die erste Sicherheit und die zweite Sicherheit jeweils einen Geldkurs und einen Frachtpreis aufweisen, der eine Paar-Handelsmaschine umfasst Für die Festlegung der Gebotsausbreitung auf dem Markt der ersten Sicherheit und der zweiten Sicherheit. Bestimmen der fragen fragen verbreitet auf dem Markt der ersten Sicherheit und der zweiten Sicherheit. Bestimmen, dass die minimale Spreizgrenze jeder Paar Handel Anfrage erfüllt ist Eine Reihe von Gebotsgebotsausbreitung und fragte fragen, dass sie sich für jede Sicherheit verbreitete. Eine Ausführung einer Transaktion zwischen einem ersten Teil des Handels der ersten Sicherheit in der einen Paarhandelsanforderung und mindestens einer Nichtpaar-Handelsanforderung, vorausgesetzt, dass das Minimum Die Spreizgrenze der einen Paarhandelsanforderung durch den Bereich der Gebotsausbreitungssphäre erfüllt ist, und die Sprechsprechverbreitung und ein Paarübergangsnetzwerk zum Anpassen eines zweiten Teils des Handels des ersten Wertes in der einen Paarhandelsanforderung und wenigstens Einen ersten Teil des Handels der zweiten Sicherheit in der einen Paarhandelsanforderung gegen die andere Paarhandelsanforderung, vorausgesetzt, dass ein Bereich der minimalen Spreizgrenze der einen Paarhandelsanforderung und der anderen Paarhandelsanforderung mit dem Bereich der genannten überlappen Bid-Spread-Spread und fragte Ask-Spread.25 Das System nach Anspruch 24, das ferner eine Verknüpfung zu einem externen Markt umfasst, wobei die Paar-Trading-Engine die Transaktion für den ersten Teil des Handels der ersten Sicherheit in der einen Paar-Handelsanforderung zur Ausführung weiterleitet In dem externen Markt über die Verbindung.26 Das System nach Anspruch 24, das ferner ein Finanzinstitut mit einem Auftragsbestand umfasst und wobei die Paarhandhabungsmaschine die Transaktion für den ersten Teil des Handels der ersten Sicherheit in der einen Paarhandelsanforderung gegenführt Das besagte bestellungsinventar.27 Das System nach Anspruch 24, wobei die Paarhandelsmaschine bestimmt, ob der Geldkurs der ersten Sicherheit in der einen Paarhandelsanforderung und der Geldpreis der zweiten Sicherheit in der einen Paarhandelsanforderung eine Spreizgrenze erfüllt, bestimmt eine Betrag der zweiten Sicherheit, die auf der Grundlage einer Gebotsgröße verkauft werden kann, die mit der zweiten Sicherheit assoziiert ist, berechnet eine äquivalente Menge des ersten Wertpapiers, die auf der Grundlage der Menge der zweiten Sicherheit, die verkauft werden kann, gekauft werden kann, die äquivalente Menge des ersten, Sicherheit auf der Grundlage von Anpassungskriterien berechnet einen Kaufpreis für den bereinigten äquivalenten Betrag der ersten Sicherheit auf der Grundlage der Spreadgrenze führt eine einleitende Bestellung aus, um den angepassten äquivalenten Betrag des ersten Wertpapiers zu dem Kaufpreis zu kaufen und führt einen Deckungsauftrag aus, um diesen Betrag zu verkaufen Der zweiten Sicherheit.28 System nach Anspruch 27, wobei die Paarhandelsmaschine eine Abdeckungsreihenfolge ausführt, um den Betrag der zweiten Sicherheit zum Angebotspreis der zweiten Sicherheit zu verkaufen.29 System nach Anspruch 27, wobei die Paarhandhabungsmaschine bestimmt, ob Die fragen Preis der ersten Sicherheit und der Ask-Preis der zweiten Sicherheit erfüllen eine Spread-Grenze bestimmt einen Betrag der ersten Sicherheit, die auf der Grundlage einer Angebotsgröße, die mit der ersten Sicherheit kalkuliert werden kann, berechnet eine entsprechende Menge der zweiten Sicherheit, die kann Verkauft werden, basierend auf dem Betrag der zweiten Sicherheit, die gekauft werden kann, stellt der äquivalente Betrag der zweiten Sicherheit auf der Grundlage von Anpassungskriterien ein Verkaufspreis für den angepassten äquivalenten Betrag der zweiten Sicherheit auf der Grundlage der Spreizgrenze führt eine einleitende Bestellung zu verkaufen Der angepasste äquivalente Betrag des zweiten Wertpapiers zum Verkaufspreis und führt eine Deckungsreihenfolge zum Kauf des Betrags der ersten Sicherheit durch. 30 System nach Anspruch 29, wobei die Paarhandhabungsmaschine eine Abdeckungsreihenfolge ausführt, um die Menge der ersten Sicherheit zu erwerben Der Ansprechpreis der ersten Sicherheit.31 System nach Anspruch 27, wobei die Anpassungskriterien einen Mindestbetrag und einen Höchstbetrag enthalten.32 System nach Anspruch 31, wobei die Paarhandhabungsmaschine die Einleitungsreihenfolge auf eine runde Losgröße umrundet.33 Die System nach Anspruch 24, wobei die Paarhandhabungsmaschine mindestens einen Teil des Handels einer der Wertpapiere in einer der Paarhandelsanforderungen in einer Vielzahl von Tranchen ausführt.34 System nach Anspruch 24, wobei die eine Paarhandelsanforderung eine erste aufweist Spreizgrenze und die andere Paarhandelsanforderung eine zweite Spreizgrenze aufweist und wobei das Paarkreuzungsnetzwerk bestimmt, dass ein Bereich der ersten Spreizgrenze und der zweiten Spreizgrenze mit einer Marktspanne überlappt, setzt ein Spread Level die Preise für die erste Sicherheit und die Zweite Sicherheit, die sich innerhalb des Marktes befinden und auf der Grundlage des Spreizniveaus basieren und den zweiten Teil des Handels der ersten Sicherheit in der einen Paarhandelsanforderung und mindestens einen ersten Teil des Handels der zweiten Sicherheit in dem einen Paar Handel übereinstimmen Anforderung gegen eine andere der Mehrzahl von Paarhandelsanforderungen auf der Grundlage der berechneten Preise.35 System nach Anspruch 34, wobei das Paarübergangsnetzwerk einen Mittelwert zwischen der ersten Spreizgrenze und der zweiten Spreizgrenze berechnet und den Spreizpegel als den Mittelwert festlegt, wenn dies der Fall ist Gemein innerhalb des Marktes verteilt ist.36 System nach Anspruch 35, wobei das Paar-Kreuzungsnetz eine Spreizmenge identifiziert, die dem Mittelwert und innerhalb des Marktspreises am nächsten liegt und das Spreizniveau als die Spreizmenge einstellt, wenn das Mittel nicht innerhalb des Marktes liegt Verbreitung.37 System nach Anspruch 24, wobei die Ein-Paar-Handelsanforderung eine erste Spreizgrenze, ein Kaufverhältnis und ein Verkaufsverhältnis aufweist, wobei die andere Paarhandelsanforderung eine zweite Spreizgrenze, ein Kaufverhältnis und ein Verkaufsverhältnis aufweist und wobei das Paar Kreuzungsnetzwerk bestimmt, dass das Kaufverhältnis und das Verkaufsverhältnis, das mit der einen Paarhandelsanforderung verbunden ist, nicht gleich dem Kaufverhältnis und dem Verkaufsverhältnis der anderen Paarhandelsanforderung entspricht und dass eine Überlappung zwischen dem Bereich der ersten Spreizgrenze und der zweiten existiert Spread-Limit und eine Marktspreizung bestimmt, dass Marktpreise existieren, die innerhalb der Überlappung sind, bestimmt einen Mismatch-Betrag in der zweiten Sicherheit, basierend auf einer Differenz zwischen dem Kaufverhältnis und dem Verkaufsverhältnis, die mit der Ein-Paar-Handelsanforderung und dem Kaufverhältnis und dem Verkauf verbunden ist Verhältnis der anderen Paarhandelsanforderung einen Kreuzbetrag für die erste Sicherheit berechnet und die zweite Sicherheit einen Kreuzungspreis für die erste Sicherheit auswählt und die zweite Sicherheit, die innerhalb der Überlappung liegt, bestimmt, dass die Fehlanpassungsmenge bei dem Kreuzungspreis für die zweite verfügbar ist Sicherheit entspricht dem zweiten Teil des Handels des ersten Wertes in der einen Paarhandelsanforderung und mindestens dem ersten Teil des Handels der zweiten Sicherheit in der einen Paarhandelsanforderung gegen die andere Paarhandelsanforderung auf der Grundlage der ausgewählten Preise und führt aus Eine Transaktion für die Fehlanpassungsmenge der zweiten Sicherheit bei dem Kreuzungspreis für die zweite Sicherheit.38 System nach Anspruch 37, wobei das Paarübergangsnetzwerk bestimmt, dass die Fehlanpassungsmenge in einem externen Markt zu dem Kreuzungspreis für die zweite Sicherheit verfügbar ist. 39. System nach Anspruch 37, wobei das Paar-Kreuzungsnetzwerk bestimmt, dass der Fehlanpassungsbetrag bei dem Kreuzungspreis für die zweite Sicherheit in dem Auftragsbestand eines Finanzinstituts verfügbar ist.40 System nach Anspruch 24, wobei die eine Paarhandelsanforderung und die Eine andere Paarhandelsanforderung eine Anzahl von Spreads anzeigt und wobei das Paarübergangsnetzwerk mit dem zweiten Teil des Handels der ersten Sicherheit in der einen Paarhandelsanforderung und wenigstens dem ersten Teil des Handels der zweiten Sicherheit in dem einen Paar Handel übereinstimmt Anforderung gegen die andere Paarhandelsanforderung, wenn die Anzahl der Spreads größer als eine minimale Anzahl von Spreads ist.41 System nach Anspruch 24, wobei die Vielzahl von Paarhandelsanforderungen mindestens einige Paarhandelsanforderungen enthält, die eine Vorliebe für die Ausführung über die Paarkreuzung anzeigen Wobei das System ferner einen Portfolio-Manager in Kommunikation mit dem Paar-Crossing-Netzwerk umfasst, wobei der Portfolio-Manager die Vielzahl von Paar-Trade-Anfragen empfängt und die mindestens einen Teil der Vielzahl von Trade-Anfragen an das Paar-Crossing-Netzwerk gemäß der Präferenz 442 leitet System nach Anspruch 24, wobei das System ferner eine Paar-Trading-Engine zum Ausführen von zumindest einigen der Vielzahl von Paar-Trade-Requests umfasst, wobei das System ferner einen Portfolio-Manager in Kommunikation mit der Paar-Trading-Engine umfasst, wobei die Vielzahl von Paar-Trade-Anfragen bei Mindestens einige Paarhandelsanforderungen, die eine Vorliebe für die Ausführung über die Paarhandhabungsmaschine anzeigen, wobei der Portfoliomanager die Mehrzahl von Paarhandelsanforderungen empfängt und die zumindest einige der Vielzahl von Handelsanforderungen an die Paarhandhabungsmaschine gemäß der Präferenz leitet.43 Die Verfahren nach Anspruch 24, wobei ein Client eine Abfrage bezüglich eines Status der Paarhandelsanforderung einreicht und der Status der Paarhandelsanforderung kontinuierlich in Echtzeit aktualisiert wird.44 Verfahren nach Anspruch 41, wobei die Präferenz von einem Client elektronisch eingereicht wird.45 Verfahren nach Anspruch 24, wobei ein Client einen gleichzeitigen Bericht empfängt, wenn die Paarhandelsanforderung gefüllt ist und der Client den Paarhandel elektronisch bestätigt.46 Ein computerlesbares Speichermedium, das Anweisungen zur Erfüllung einer Paarhandelsanforderung speichert, die, wenn sie von einem Computer ausgeführt wird, Zu veranlassen, eine Vielzahl von Paarhandelsanforderungen zu erfassen, die eine Paarhandelsanforderung und eine andere Paarhandelsanforderung umfassen, wobei jede Paarhandelsanforderung eine Anforderung zum Handel einer ersten Sicherheit, eine Anforderung zum Handel einer zweiten Sicherheit und eine Anforderung an umfasst Handel sagte erste Sicherheit und sagte zweite Sicherheit mit einer minimalen Spreizgrenze, und wobei die erste Sicherheit und die zweite Sicherheit jeweils einen Geldkurs und einen fragen Preis haben. Bestimmen Sie das Gebot Gebot auf dem Markt der ersten Sicherheit und die zweite Sicherheit zu verbreiten. Bestimmen die fragen fragen verbreiten auf dem markt der ersten sicherheit und die zweite sicherheit. dest bestimmen, dass die minimale verbreitung begrenzung jeder paar handelsanforderung durch eine reihe der geboten gebot verbreitung erfüllt ist und die frage fragen verbreiten. execute eine transaktion zwischen einem ersten Teil des Handels des ersten Wertpapiers in der einen Paarhandelsanforderung und mindestens einer Nichtpaar-Handelsanforderung, vorausgesetzt, dass die minimale Spreizgrenze der einen Paarhandelsanforderung durch den Bereich der Gebotsgebotsausbreitung erfüllt ist und die Frage fragen Verbreiten und einen zweiten Teil des Handels der ersten Sicherheit in der einen Paarhandelsanforderung und mindestens einen ersten Teil des Handels der zweiten Sicherheit in der einen Paarhandelsanforderung gegen die andere Paarhandelsanforderung, vorausgesetzt, dass ein Bereich of the minimum spread limit of said one pair trade request and said another pair trade request overlaps with said range of said bid bid spread and said ask ask spread. CROSS-REFERENCE TO RELATED APPLICATION. This application claims the benefit of the filing date of US provisional application Ser No 60 334,163 entitled Method and System for Trading Pairs of Securities, that was filed on Nov 29, 2001, the contents of which are incorporated by reference herein. The following invention relates to a system and method for trading securities and, in particular, for a system and method of trading securities in pairs. A recognized strategy for trading securities is known as pair-trading Pair-trading is a non-directional investment strategy in which the investor identifies two securities having similar characteristics and the securities are currently trading at a price relationship that is out of their historical trading range The investor exploits the price relationship between the securities by buying the undervalued security while short-selling the overvalued security Because pair-trading is a market-neutral strategy, it is a particularly desirable strategy for investing in volatile markets. One context in which pair trading is useful is where an investor desires to take advantage of an arbitrage opportunity resulting from a merger between two companies For example, Company A has announced a definitive agreement to acquire Company T in which case Company T shareholders will receive 0 5 shares of Company A stock for each share of Company T stock they own The investor desires to capture the spread between the offered consideration 0 5 shares of A and the price of T stock To do this, the investor buys shares in T stock and sells shares of A stock. For instance, if stock T is trading at 28 per share and stock A is trading at 60 per share, then the investor may execute a trade for 200,000 spreads by buying 200,000 shares of T stock and selling 100,000 shares of A stock After the merger takes place, the investor will cover the short position in stock A with the 100,000 shares of A stock the investors receives in exchange of the 200,000 shares the investor held of stock T Thus, by executing the pair trade, the investor locks in a 400,000 profit assuming that the merger goes through The process of executing a pair trade thus includes executing individual trades directed to each leg of the pair trade request An example of a system for executing trades for filling a pair trade request is the Quantex system from ITG of 380 Madison Avenue New York, NY 10017.A challenge in implementing a pair trade is to find a counterparty for a particular position an investor desires to establish while minimizing leg risk Typically, a large pair trade is performed off the market as a private transaction negotiated by a financial institution that services large clients For example, if an investor desires to execute a pair trade betting that a proposed merger between two companies will go through, the investor would approach a financial institution seeking an investor that is willing to bet against the merger The financial institution then acts as an intermediary between the two investors in which the investors establish equal and opposite positions in the stock of the proposed merger partners thereby completing the pair trade Thus by matching two pair trade requests so that the transactions associated with each of the pair trade legs are executed simultaneously, neither investor is exposed to leg risk that would otherwise result for the period of time between execution of the first leg and the second leg of the pair trade. There are numerous drawbacks associated with the prevalent pair-trading practice First, pair-trading is typically limited to clients of large financial institutions that have the ability to identify suitable counterparties for a particular pair trade This is especially the case when the pair trade involves a large amount of stock or illiquid stocks in which the only way to execute the trade and minimize leg risk is via an off the market transaction negotiated by a financial institution Also, because a pair-trade is typically negotiated by the parties with a financial institution as an intermediary, the process is often slow and inefficient Furthermore, pair-trading under current practice is generally best suited for large clients seeking to establish large positions thereby providing the financial institutions with the economic incentive to execute the transaction Smaller clients, however, must rely on the markets for executing pair trades, which is unsuitable for illiquid stocks and also results in increased leg risk. Accordingly, it is desirable to provide a system and method for trading securities in pairs. SUMMARY OF THE INVENTION. The present invention is directed to overcoming the drawbacks of the prior art pair trading practices Under the present invention a method is provided for fulfilling a pair trade request and includes the steps of receiving a plurality of pair trade requests executing a transaction for a first portion of one of the plurality of pair trade requests and matching a second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests. In an exemplary embodiment, the method includes the step of executing a transaction for a first portion of one of the plurality of pair trade requests in an external market. In another exemplary embodiment, the method includes the step of executing a transaction for a first portion of one of the plurality of pair trade requests against the order inventory. In yet another exemplary embodiment, the pair trade request includes a first security having a bid price and an ask price and a second security having a bid price and an ask price, and the method includes the steps of determining whether the bid price of the first security and the bid price of the second security meet a spread limit determining an amount of the second security that can be sold based on a bid size associated with the second security calculating an equivalent amount of the first security that can be bought based on the amount of the second security that can be sold adjusting the equivalent amount of the first security based on adjustment criteria calculating a purchase price for the adjusted equivalent amount of the first security based on the spread limit executing an initiating order to buy the adjusted equivalent amount of the first security at the purchase price and executing a covering order to sell the amount of the second security. In still yet another exemplary embodiment, the method includes the step of executing a covering order to sell the amount of the second security at the bid price of the second security. In an exemplary embodiment, the method includes the steps of determining whether the ask price of the first security and the ask price of the second security and or the bid price of the first security and the bid price of the second security meet a spread limit determining an amount of the first security that can be bought based on an offer size associated with the first security calculating an equivalent amount of the second security that can be sold based on the amount of the second security that can be bought adjusting the equivalent amount of the second security based on adjustment criteria calculating a selling price for the adjusted equivalent amount of the second security based on the spread limit executing an initiating order to sell the adjusted equivalent amount of the second security at the selling price and executing a covering order to purchase the amount of the first security. In another exemplary embodiment, the method includes the step of executing a covering order to purchase the amount of the first security at the ask price of the first security. In yet another exemplary embodiment, the adjustment criteria include a minimum amount and a maximum amount. In still yet another exemplary embodiment, the method includes the step of rounding the initiating order to a round lot size. In an exemplary embodiment, the method includes the step of executing a first portion of one of the plurality of pair trade requests in a plurality of tranches. In another exemplary embodiment, the one of the plurality of pair trade requests and the another of the plurality of pair trade requests include a first security and a second security, the one of the plurality of pair trade requests has a first spread limit and the another of said plurality of trade requests has a second spread limit and wherein the method includes the steps of determining that a range of the first spread limit and the second spread limit overlaps with a market spread setting a spread level calculating prices for the first security and the second security that are within the market spread and based on the spread level and matching the second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests based on the calculated prices. In yet another exemplary embodiment , the method includes the steps of calculating a mean between the first spread limit and the second spread limit and setting the spread level as the mean if the mean is within the market spread. In still yet another exemplary embodiment, the method includes the step of identifying a spread amount that is closest to the mean and within the market spread and setting the spread level as the spread amount if the mean is not within the market spread. In an exemplary embodiment, the one of the plurality of pair trade requests and the another of the plurality of pair trade requests include a first security and a second security, the one of the plurality of pair trade requests has a first spread limit, a buy ratio and a sell ratio, the another of the plurality of trade requests has a second spread limit, a buy ratio and a sell ratio and the method includes the steps of determining that the buy ratio and the sell ratio associated with the one of the plurality of trade requests does not equal the buy ratio and the sell ratio of the another of the plurality of trade requests and that an overlap exists between range of the first spread limit and the second spread limit and a market spread determining that market prices exist that are within the overlap determining a mismatch amount in the second security based on a difference between the buy ratio and the sell ratio associated with the one of the plurality of trade requests and the buy ratio and the sell ratio of the another of the plurality of trade requests calculating a cross amount for the first security and the second security selecting a crossing price for the first security and the second security that is within the overlap determining that the mismatch amount is available at the crossing price for the second security matching the second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests based on the calculated prices and executing a transaction for the mismatch amount of the second security at the crossing price for the second security. In another exemplary embodiment, the method includes the step of determining that the mismatch amount is available in an external market at the crossing price for the second security. In yet another exemplary embodiment, the method is performed by a financial institution having order inventory and includes the step of determining that the mismatch amount is available in the order inventory at the crossing price for the second security. In still yet another exemplary embodiment, the one of the plurality of pair trade requests and the another of the plurality of pair trade requests indicate a number of spreads and the method includes the step of matching a second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests if the number of spreads is greater than a minimum number of spreads. In an exemplary embodiment, the method includes the step of receiving a preference for filling at least some of the plurality of trade requests via the step of executing a transaction for a first portion of one of the plurality of pair trade requests, described above. In another exemplary embodiment, the method includes the step of receiving a preference for filling at least some of the plurality of trade requests via the step of matching a second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests, described above. Under the present invention, a method for fulfilling a pair trade request is provided and includes the steps of receiving a plurality of pair trade requests and matching at least a portion of one of the plurality of pair trade requests against another of the plurality of pair trade requests. Under the present invention, a system for fulfilling a pair trade request is provided, the system receiving a plurality of pair trade requests and includes a pair trading engine for executing a transaction for a first portion of one of the plurality of pair trade requests The system also includes a pair crossing network for matching a second portion of said one of the plurality of pair trade requests against another of the plurality of pair trade requests. In an exemplary embodiment, the system includes a link to external markets and wherein the pair trading engine executes the transaction for the first portion of one of the plurality of pair trade requests in the external markets. In another exemplary embodiment, the system includes a financial institution having an order inventory and wherein the pair trading engine executes the transaction for the first portion of one of the plurality of pair trade requests against the order inventory. In yet another exemplary embodiment , the pair trade request includes a first security having a bid price and an ask price and a second security having a bid price and an ask price, and wherein the pair trading engine determines whether the bid price of the first security and the bid price of the second security meet a spread limit determines an amount of the second security that can be sold based on a bid size associated with the second security calculates an equivalent amount of the first security that can be bought based on the amount of the second security that can be sold adjusts the equivalent amount of the first security based on adjustment criteria calculates a purchase price for the adjusted equivalent amount of the first security based on the spread limit executes an initiating order to buy said adjusted equivalent amount of the first security at the purchase price and executes a covering order to sell the amount of the second security. In still yet another exemplary embodiment, the pair trading engine executes a covering order to sell the amount of the second security at the bid price of the second security. In an exemplary embodiment , the pair trading engine determines whether the ask price of the first security and the ask price of the second security meet a spread limit determines an amount of the first security that can be bought based on an offer size associated with the first security calculates an equivalent amount of the second security that can be sold based on the amount of the second security that can be bought adjusts said equivalent amount of the second security based on adjustment criteria calculates a selling price for the adjusted equivalent amount of the second security based on the spread limit executes an initiating order to sell the adjusted equivalent amount of the second security at the selling price and executes a covering order to purchase the amount of the first security. In another exemplary embodiment, the pair trading engine executes a covering order to purchase the amount of the first security at the ask price of the first security. In yet another exemplary embodiment, the pair trading engine rounds the initiating order to a round lot size. In still yet another exemplary embodiment, the pair trading engine executes a first portion of one of the plurality of pair trade requests in a plurality of tranches. In an exemplary embodiment, the one of the plurality of pair trade requests and the another of the plurality of pair trade requests include a first security and a second security, the one of the plurality of pair trade requests has a first spread limit and the another of the plurality of trade requests has a second spread limit and wherein the pair crossing network determines that a range of the first spread limit and the second spread limit overlaps with a market spread sets a spread level calculates prices for the first security and the second security that are within the market spread and based on the spread level and matches the second portion of said one of the plurality of pair trade requests against another of the plurality of pair trade requests based on the calculated prices. In another exemplary embodiment, the pair crossing network calculates a mean between the first spread limit and the second spread limit and sets the spread level as the mean if the mean is within the market spread. In yet another exemplary embodiment, the pair crossing network identifies a spread amount that is closest to the mean and within the market spread and sets the spread level as the spread amount if the mean is not within the market spread. In still yet another exemplary embodiment, the one of said plurality of pair trade requests and the another of the plurality of pair trade requests include a first security and a second security, the one of the plurality of pair trade requests has a first spread limit, a buy ratio and a sell ratio, the another of the plurality of trade requests has a second spread limit, a buy ratio and a sell ratio and wherein the pair crossing network determines that the buy ratio and the sell ratio associated with the one of the plurality of trade requests does not equal the buy ratio and the sell ratio of the another of the plurality of trade requests and that an overlap exists between range of the first spread limit and the second spread limit and a market spread determines that market prices exist that are within the overlap determines a mismatch amount in the second security based on a difference between the buy ratio and the sell ratio associated with the one of the plurality of trade requests and the buy ratio and the sell ratio of the another of the plurality of trade requests calculates a cross amount for the first security and the second security selects a crossing price for the first security and the second security that is within said overlap determines that the mismatch amount is available at the crossing price for the second security matches the second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests based on the calculated prices and executes a transaction for the mismatch amount of the second security at the crossing price for the second security. In an exemplary embodiment, the pair crossing network determines that the mismatch amount is available in an external market at the crossing price for the second security. In another exemplary embodiment, the pair crossing network determines that the mismatch amount is available in the order inventory at the crossing price for the second security. In yet another exemplary embodiment, the one of said plurality of pair trade requests and the another of the plurality of pair trade requests indicate a number of spreads and wherein the pair crossing network matches a second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests if the number of spreads is greater than a minimum number of spreads. In still yet another exemplary embodiment, the plurality of pair trade requests include at least some pair trade requests indicating a preference for execution via said pair crossing network, and the system further includes a portfolio manager in communications with the pair crossing network, the portfolio manager receiving the plurality of pair trade requests and routing the at least some pair trade requests to the pair crossing network according to the preference. In an exemplary embodiment, the system includes a pair trading engine for executing at least some of the plurality of pair trade requests, further includes a portfolio manager in communications with the pair trading engine and wherein the plurality of pair trade requests include at least some pair trade requests indicating a preference for execution via the pair trading engine, the portfolio manager receiving the plurality of pair trade requests and routing the at least some of the plurality of trade requests to the pair trading engine according to the preference. Under the present invention, a system for fulfilling a pair trade request is provided, wherein the system receives a plurality of pair trade requests and includes a pair crossing network for matching at least one of the plurality of pair trade requests against another of the plurality of pair trade requests. Accordingly, a method and a system are provided for trading pair securities. The invention accordingly comprises the features of construction, combination of elements and arrangement of parts that will be exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims Other features and advantages of the invention will be apparent from the description, the drawings and the claims. DESCRIPTION OF THE DRAWINGS. For a fuller understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawings, in which. FIG 1 is a block diagram of a system for trading securities in pairs according to the present invention. FIG 2 is a flowchart of the steps a pair trading engine included in the system of FIG 1 applies to fill a pair trade request. FIG 3 is a flowchart of the steps a pair crossing network included in the system of FIG 1 applies to fill a pair trade request. FIG 4 is a flowchart of a process by which the pair crossing network of the system of FIG 1 fills imperfectly matched orders and. FIG 5 is a graph for identifying the market prices for two securities that meet the required spread limits. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS. Referring now to FIG 1 there is shown a block diagram of a system 1 for trading securities in pairs according to the present invention System 1 receives pair trade requests from clients operating client access devices 7 and attempts to fill the pair trade requests according to the parameters associated with the particular pair trade request System 1 includes two different subsystems for filling pair trade requests a pair trading engine 3 and a pair crossing network 5 As will be described below, pair trading engine 3 receives a pair trade request and attempts to fill in whole or in part the trade request by executing the appropriate trades in an external market 13 that may include, by way of non-limiting example, the New York Stock Exchange, the NASDAQ or any other financial market Pair trading engine 3 may also fill in whole or in part a pair trade request by executing a transaction against order inventory 11 of non-pair trade requests controlled by the financial institution that is operating system 1 In addition, pair trading engine may also fill in whole or in part a pair trade request by forwarding the trade request to pair crossing network 5 for matching with other pair trade requests. Likewise, pair crossing network 5 receives a pair trade request and fulfills in whole or in part the request by matching it against another pair trade request received by pair crossing network 5 by matching the request against inventory 11 controlled by the financial institution and or by forwarding the trade request to pair trading engine 3 for execution in external markets 13. System 1 also includes a portfolio manager 9 that may be, for example, a software program executing on a computer system that receives the pair trade requests from client access device 7 and presents the trade request to either pair trading engine 3 pair crossing network 5 or both, depending on the trade parameters set by the client Also, the client may query portfolio manager 9 regarding the status of any pair trade request the client has presented to system 1.In operation, system 1 may fulfill a pair trade request either using pair trading engine 3 or pair crossing network 5 or a combination of the two For example, a pair trade request received by system 1 may be completely filled by pair trading engine 3 as follows. Assume a case where XYZ is taking over ABC and is offering 0 575 shares of XYZ for each ABC share and investor Arb wants to invest in the price difference between ABC stock and XYZ stock To take advantage of the price difference, Arb wants to lock in the difference between the value offered 0 575 XYZ stock and the value of ABC stock by buying ABC stock and selling XYZ stock subject to the condition that ABC 0 575 XYZ 1 19 ie Arb desires to capture a 1 19 difference between XYZ s takeover offer and ABC s share price. In order to fill this pair trade, Arb presents a pair trade request to portfolio manager 9 using client access device 7 The pair trade request typically includes a number of parameters that define the pair trade and that also may be used by portfolio manager 9 in determining how the pair trade request is to be filled Arb typically indicates in the trade request the number of spreads the Arb desires to invest in and also provides a minimum and maximum share amount that he is willing to trade per tranche. For example, Arb may indicate a desire to invest in 100,000 spreads and may only wish to trade the spread 3,000-8,000 shares at a time Arb generally sets this tranche size range based on the liquidity and volatility of ABC stock and XYZ stock Arb may set a larger minimum tranche size if ABC stock and XYZ stock are fairly liquid stocks because higher liquidity increases the likelihood that a larger tranche size will be executed Arb may set a lower maximum tranche size if XYZ stock and ABC stock are volatile stocks so as to limit the leg risk associated with executing a pair trade. Yet another pair trade parameter Arb provides is the spread limit in the above case 1 19 which is the amount Arb desires to capture in the trade Arb does not have to provide, however, the discrete prices at which trades for ABC and XYZ stock are to be executed as these prices are calculated by pair trading engine 3 and or pair crossing network 5 , as will be described below. Referring now to FIG 2 there is shown a flowchart describing the steps pair trading engine 3 applies to fill a pair trade request The flowchart in FIG 2 is based on the above example and the market data listed in Table 1 below. Initially, in Step 201 pair trading engine 3 determines whether the bid bid prices or ask ask prices of ABC and XYZ stock, respectively, meet the spread limit requirement of the particular pair trade request In this case the bid bid spread is 1 4375 122 50 0 575 69 and the ask ask spread is 1 1969 122 625 0 575 69 3125 so that each spread is less than the spread limit of 1 19, as is required for this particular trade Once it is determined that either the bid bid spread or the ask ask spread meets the spread limit, then in Step 202 it is determined as is indicated in Table 1 how much XYZ stock can be sold at the bid and how much ABC stock can be bought at the ask In an exemplary embodiment, the client may specify whether the bid bid spread, the ask ask spread or either the bid bid or the ask ask spread must exceed the indicated spread limit for a transaction to proceed If neither the bid bid spread nor the ask ask spread meets the spread limit, the process waits a period of time for example 0 10 seconds and returns to Step 201 to again test whether the bid bid spread or the ask ask spread meets the spread limit. Next , in step 203 an equivalent amount of stock that can be sent into the market ie bought sold in the market is calculated for a spread based on the bid bid price spread and or the ask ask price spread that meets the spread limit In this example, if a maximum of 10,000 shares of XYZ stock can be sold into the market ie the XYZ bid size then, based on the ABC XYZ ratio of 1 0 575 in this case , a total of 17,391 10,000 0 575 shares of ABC stock are to be bought in order to execute a balanced pair trade Likewise, if a maximum of 1,500 shares of ABC stock can be bought in the market ie the ABC ask size , then, based on the ABC XYZ ratio of 1 0 575 in this case , a total of 863 1500 0 575 shares of XYZ stock are to be sold in order to execute a balanced pair trade. Next, in Step 204 the pair trade share amounts calculated in Step 203 are adjusted to conform to the wave maximum and minimum parameters ie the maximum minimum tranche size included in the pair trade request as well as market round lot limits In the above example, the amount of ABC shares to be bought that was calculated based on the XYZ bid size ie 17,391 is first rounded to an even lot size ie 17,400 and then reduced to the maximum tranche size of 8000 Also, the amount of XYZ shares to be offered that was calculated based on the ABC ask size ie 863 is first rounded to an even lot size ie 900 and then increased to 1,700 shares to meet the minimum tranche size of 3000 3000 0 575 1777 In an exemplary embodiment the minimum and maximum tranche size is scaled by the particular ratio for example, in the above case, the tranche sizes for XYZ stock is scaled by 0 575 In another embodiment, the maximum minimum tranche size is used for each security in the pair trade request without scaling In yet another exemplary embodiment, the pair trade request includes a separate maximum minimum tranche for each security. Once the share amounts for the pair trade are calculated, in Step 205 the share prices that are needed to meet the spread limit of the pair trade request are calculated For example, for a pair trade based on the bid bid price spread, in order to meet the spread limit of 1 19 credit, the price at which ABC stock is to be bid should be no greater than 69 2475 122 50 0 575 1 19 a share Likewise, for a pair trade based on the ask ask price spread, in order to meet the spread limit, the price at which XYZ stock is to be offered should be greater than or equal to 122 6130 69 3125 1 19 0 575 a share. Next, once the pair trade share amounts and share prices have been calculated, in Step 206 pair trading engine 3 sends initiating orders to external markets 13 in order to fill the pair trade request The initiating orders may include an initiating order for executing a pair trade based on the bid bid spread in this case a bid for 8,000 shares of ABC stock at 69 2475 and or an initiating order for executing a pair trade based on the ask ask spread in this case an offer of 1,700 shares of XYZ stock at 122 6130.Finally, as the initiating orders sent to external markets 13 in Step 207 get filled, pair trading engine 3 automatically sends into the market the covering side of the pair trade So, for example, as the initiating order of buying 8,000 shares of ABC stock at 69 2475 gets filled, pair trading engine 3 sends an order to external markets 13 to sell 4,600 8,000 0 575 shares of XYZ stock at 122 50. In an exemplary embodiment, the client s pair trade request includes threshold amounts that indicate the amount of variance in stock price and or share amount the client is willing to absorb For example, if in the process of covering the initiating order the price of XYZ stock dips to 122 49 in which case the spread limit of the pair trade would drop to 1 18 , then pair trading engine 3 would still sell XYZ stock at the price of 122 49 if the 0 01 difference was within the threshold amount included in the pair trade request Similarly, the pair trade request may include threshold amounts for any other pair trade parameter, including by way of non-limiting example, the number of spreads to be purchased and the tranche sizes If, however, a particular threshold amount indicated by the client is exceeded for any given pair trade parameter, then pair trading engine 3 would attempt to cancel the initiating order and or the covering order that may be possible if the orders have not yet reached the market or have not yet been filled In such a case , pair trading engine 3 would then repeat the above analysis for determining suitable initiating and cover orders. To fill a pair trade request, pair trading engine 3 executes trades utilizing the method described above Typically, pair trading engine 3 tranches a pair trade request and trades piece-meal in external markets 13 In certain cases, however, it may be difficult to fill a trade request by executing several transactions in external markets 13 either because the pair trade request is for a very large number of spreads or includes stocks that are illiquid in which cases pair trading engine 3 may be ineffective in filling the pair trade request Also, in certain situations, a client wishing to remain anonymous may indicate in the pair trade request a preference that no orders be sent to external markets 13 In these circumstances, portfolio manager 9 may route the particular pair trade request to pair crossing network 5.Referring now to FIG 3 there is shown a flowchart illustrating the steps pair crossing network 5 applies to fill a pair trade request The flowchart in FIG 3 is based on the above example and the market data listed in Table 2 below. Spread Limit as defined by 0 575 XYZ - ABC. Continuing the previous example, assume the pair trade request issued by Arb for 100,000 spreads was half-filled by pair trading engine 3 Also, assume that system 1 receives a pair trade request from Antiarb that indicates a desire to sell 30,000 shares of ABC and buy 17,200 shares a ratio of 1 0 575 and also indicates a spread limit of 1 30 i e ABC 0 575XYZ 1 30 In this case Arb and Antiarb s orders are complimentary in the primary order elements securities, ratios and buy versus sell Also, Antiarb is willing to pay 0 11 per spread more than Arb is demanding from the marketplace Based on these parameters, there is an opportunity for Arb s and Antiarb s trade requests to be filled via pair crossing network 5.If Antiarb s pair trade request was marked for trading by pair trading engine 3 then portfolio manager 9 sends Antiarb s order to pair trading engine 3 for execution Pair trading engine 3 then sends the parameters of Antiarb s trade request, as well as all orders waiting for execution in pair trading engine 3 to pair crossing network 5 Pair crossing network 5 will recognize as described above that there is a crossing opportunity between Arb s order and Antiarb s order In this case, pair crossing network 5 then directs pair trading engine 3 to suspend the execution of Antiarb s order in the amount that can be crossed by pair crossing network 5 30,000 spreads in this case In addition, pair trading engine 3 routes a cross amount of 30,000 spreads from Arb s order to pair crossing network 5 for crossing against Antiarb s order At this point, the pair crossing network 5 crosses the Antiarb order against a portion of Arb s order, as follows. Assume the prevailing market conditions at the time of the cross are as shown in Table 3 Furthermore, Table 3 indicates the XYZ Ratio-Adjusted Value for both the bid and ask prices based on the conversion ratio of 1 0 575 Based on the XYZ Ratio-Adjusted Values, a Bid Ask Spread Range i e the spread provided for a cross between the bid price of ABC stock and the XYZ Ratio-Adjusted ask price of 1 3863 is calculated and an Ask Bid Spread Range i e the spread provided for a cross between the ask price of ABC stock and the XYZ Ratio-Adjusted bid price of 1 05 is calculated. To perform the cross, in Step 301 pair crossing network 5 first determines whether the range of spread limits associated with Arb s and Antiarb s trade requests i e 1 30- 1 19 coincides with the range of the prevailing market spread 1 3863- 1 05 In this example, the range of spread limits does coincide with the prevailing market spread because at least a portion of the spread limit range overlaps with a portion of the market spread Thus, a cross can occur. Next, in Step 302 pair crossing network 5 calculates the mean of Arb s and Antiarb s spread order limit which is 1 30 1 19 2 1 245 and determines whether the mean is within the range of the market spread i e 1 3863- 1 05 If it is, then in Step 303 pair crossing network 5 calculates the prices at which to cross The prices must be within the current markets for ABC stock and XYZ stock, and satisfy market uptick requirements for short sales , and provide a spread that is equal to the spread level calculated above For example, with the inside market for ABC stock at 70 00-70 25 and the inside market for XYZ stock at 124 00-124 15, a cross price of 70 11 for ABC stock and 124 096 for XYZ stock provides the spread of 1 2452 thereby meeting the requirement of both Arb s and Antiarb s trade request Finally, in Step 304 pair crossing network 5 crosses 30,000 shares of ABC stock at 70 11 with Arb buying and Antiarb selling and 17,200 shares of XYZ at 124 096 with Arb selling and Antiarb buying. If it is determined in Step 302 that the mean of Arb s and Antiarb s spread order limits does not fall within the range of the market spread, then in Step 305 the spread closest to the mean of the two spread limits that is also within the market spread is calculated For example, if the market spread is 1 3863- 1 28, then the mean of the two spread limits 1 245 is not within the market spread In such a case, 1 28 is selected as the spread level that is closest to the mean and within the market spread In an exemplary embodiment, the spread level at which Arb and Antiarb cross can be determined in any other suitable manner as long as the spread level is within the market spread and within the range of spread limits indicated in the pair trade requests. Once the spread level is determined, the method proceeds to Step 303 in which pair crossing network 5 calculates prices to cross at that are within the current markets for ABC stock and XYZ stock and that meet the calculated spread level In the case where the calculated spread level is 1 28, the cross will occur at a price of 70 08 for ABC stock and 124 1043 for XYZ stock Finally, the method proceeds to Step 304 in which pair crossing network 5 performs the cross between Arb and Antiarb. Once a pair trade request is filled or partially filled , the transaction details are reported to portfolio manager 9 and made available to the client operating client access device 7.In the previous example, pair crossing network 5 crosses orders in which both Arb and Antiarb desire to trade the same pair of securities in the same ratio In an exemplary embodiment, pair crossing network 5 executes a cross between two pair trade requests that are not perfectly matched. For example, assume that pair crossing network 5 receives the pair trade requests as shown in Table 4 Note that these two pair trade requests are imperfectly matched because each trade request uses a different ratio between ABC and XYZ stock. Arb s Spread Limit is defined by 0 575 XYZ - ABC. Antiarb s Spread Limit is defined by 0 6 XYZ - ABC. Also, assume the market in ABC and XYZ stocks at the time the pair trade requests are received by pair crossing network 5 is as described in Table 5 below. Arb s XYZ Ratio-Adjusted Value. Arb s Dollar Range. AntiArb s XYZ Ratio-Adjusted Value. AntiArb s Dollar Range. Referring now to FIG 4 there is shown a flowchart illustrating a process by which pair crossing network 5 fills these imperfectly matched order First, in Step 401 pair crossing network 5 determines whether Arb s buy security equals Antiarb s sell security and whether Arb s sell security equals Antiarb s buy security If both conditions are not met, then a cross between the two orders cannot occur If the two conditions are met, then in Step 402 it is determined whether Arb s buy ratio equals Antiarb s sell ratio and whether Arb s sell ratio equals Antiarb s buy ratio If these ratios are the same, then pair crossing network 5 proceeds to cross the two orders as described in the example above Note that for a cross to occur at this stage does not require the ratios themselves to match but rather that the ratios of the ratios match for e g a ratio of 2 3 matches a ratio of 0 667 1.If, however, the two ratios are not equal as in this case where Arb s sell ratio does not equal Antiarb s buy ratio , then in Step 403 pair crossing network determines whether there is an overlap between Arb s and Antiarb s spread limit that also falls within the bid ask market for ABC and XYZ stock To make such a determination, pair crossing network 5 calculates whether there are market prices for both ABC and XYZ stock that satisfy the following inequalities L 1 Ratio A ABC Ratio B XYZ and 1 L 2 Ratio C ABC Ratio D XYZ 2 Where L1 is Arb s spread limit of 1 19 credit, L2 is Antiarb s spread limit of 4 40 debit, RatioA is Arb s buy ratio of 1 1, RatioB is Arb s sell ratio of 1 0 575, RatioC is Antiarb s sell ratio of 1 1 and RatioD is Antiarb s buy ratio of 1 0 6.Referring now to FIG 5 there is shown a graph 51 that depicts market prices for ABC and XYZ stock that meet the spread limits of Arb and Antiarb In graph 51 the x-axis represents the prices for XYZ stock while the y-axis represents the prices for ABC stock Graph 51 includes a shaded area 53 that is the universe of market prices for ABC and XYZ stock that could satisfy the spread trade involving those stocks Also included in graph 53 is a spreadlimit line L1 inequality 1 , above that represents the spread limit associated with Arb and a spread limit line L2 inequality 2 , above that represents the spread limit associated with Antiarb Thus, the solution set of market prices that satisfies inequalities 1 and 2 is the portion of dark shared area 53 that falls between spread limit line L1 and spread limit line L2 In this example, a cross at a share price for ABC of 70 14 and a share price of 124 15 for XYZ stock meets the investor s spread limits and falls within the market prices for ABC and XYZ stock. If it is determined that no share prices for both ABC and XYZ stock satisfy Arb s and Antiarb s spread limits, then no cross can occur If such share prices do exist, then in Step 404 it is determined which of the investors desires to transact in fewer shares of ABC stock and a mismatch in share amounts caused by the differing ratios is determined In our example, Antiarb desires to sell fewer ABC shares than Arb desires to buy 30,000 vs 50,000 Then, in Step 405 pair crossing network 5 determines the number of XYZ shares that can be crossed between Arb and Antiarb based on the maximum amount of ABC shares that can be crossed 30,000 in this example Based on the Antiarb ABC order quantity of 30,000 shares, the maximum number of XYZ shares that Arb will cross with Antiarb is.30 000 Arb XYZ Ratio Arb ABC Ratio 30 000 0 575 1 17 300 17 250 rounded to an even lotsize. While the maximum quantity of XYZ shares that Arb will cross is 17,300, Antiarb s trade request indicates a desire to cross 18,000 shares To overcome this imbalance, in Step 406 pair crossing network 5 is in communications with external markets 13 for determining whether the excess 700 XYZ shares needed to satisfy Antiarb s trade request can be transacted for in external markets 13 In an exemplary embodiment, pair crossing network 5 makes this determination by issuing a query to pair trading engine 3 as to whether 700 shares of XYZ stock can be bought in external markets 13 Because, as indicated in Table 5, 3,000 shares of XYZ stock are offered at 124 15, pair trading engine 3 responds to pair crossing network 5 that the 700 shares needed to balance the cross between Arb and Antiarb are available from external markets 13 at 124 15.Next, in Step 407 pair crossing network calculates the cross prices that are necessary such that Arb and Antiarb achieve their respective spread limits while also incorporating the excess 700 shares of XYZ stock that must be purchased from external markets 13 at 124 15 to satisfy Antiarb s trade request An example of such cross prices that meet these criteria is a price of 70 14 for ABC stock and a price of 124 15 for XYZ stock. Once the cross prices are calculated, in Step 408 pair crossing network 5 crosses 30,000 shares of ABC stock and 17,300 shares of XYZ stock between Arb and Antiarb and also buys 700 shares of XYZ stock at 124 15 in external markets 13 on behalf of Antiarb Thus, both Arb and Antiarb s pair trade requests are satisfied. Alternatively, the entire 18,000 shares of XYZ stock may be crossed thereby fully satisfying Antiarb s trade request In such a case, the ratio mismatch is addressed by Arb purchasing an additional 1200 700 0 575 rounded to a lotsize shares of ABC stock from external market 13 or from firm inventory 11.Once the trade is completed, the details of the transaction are provided to portfolio manager 9 to report the transaction details to the investors. In an exemplary embodiment, a pair order or portion thereof may be filled against an internal inventory 11 of trade requests maintained by the financial institution operating system 1 For example, in the previous example in which an excess of 700 shares of XYZ stock needs to be purchased in order for a match i e cross between Arb and Antiarb s trade requests to occur, instead of determining whether the 700 shares are available in external markets 3 pair crossing network 5 examines firm inventory 11 to determine whether the shares are available at the required price Likewise, in cases where pair trading engine 3 desires to execute a pair trade based on orders to be sent to external markets 13 pair trading engine 3 may first determine whether the order can be filled, in whole or in part, using trade requests pending in firm inventory 11 Generally, the advantages of filling an order using pending trade requests in firm inventory 11 is that execution is faster, transaction costs are lower and leg risk is minimized. In another exemplary embodiment, a client s pair trade request may also include a minimum number of spreads that can be traded in pair crossing network 5 Also, pair crossing network 5 may be designed to require a minimum share amount for a cross to occur A minimum number of spreads that can be traded may be provided in order to reduce the distractions and booking costs associated with numerous smaller trades that may exceed the benefits of a de minimis fill. In another exemplary embodiment, portfolio manager 9 publishes the inside cross market for any pair that a client has selected for crossing in pair crossing network 5 In still another exemplary embodiment, the client has the option for each pair trade selected for crossing in pair crossing network 5 to designate that the order should be reflected in the published inside cross market This inside cross market consists of the tightest spread bid and offer and corresponding bid size and offer size from all client pair orders pending in pair crossing network 5 In this way, a client can assess the likelihood and timing of a pair trade request being filled by pair crossing network 5 Also, by publishing the client s spread interest, others seeking liquidity can trade at the client s level. In an exemplary embodiment, the client can designate each pair order designated for pair trading engine 3 and or pair crossing network 5 for Broker Negotiation If Broker Negotiation is designated, the client s broker-dealer sales representative is notified of the client s spread order thereby prompting the broker-dealer to solicit a complementary, agency order from another client The client may also designate each pair order for Broker Facilitation in which case the client allows the broker-dealer to act principally to fill the client s order. In summary, the advantages to a client of using pair trading engine 3 is that pair trading engine 3 allows the client to trade a spread order while limiting leg risk or the risk of missing a targeted spread level This is accomplished by breaking the total order into tranches of sizes proportionate to the market, subject to user minimums and maximums, that can be traded in external markets 13 or against firm inventory 11 Orders executed via pair trading engine 3 however, are typically of a lower traded volume because trading is constrained to the liquidity available in the market In contrast, trades executed via pair crossing network 5 are not constrained by market liquidity and do not have to be tranched to minimize leg risk In particular, the benefits of filling a pair trade request via pair crossing network 5 are as follows. Elimination of Leg Risk Pair crossing network 5 potentially provides a deeper well of liquidity because the trades are brokered, as a spread, directly between spread investors via a central clearing facility Moreover, the introduction and use of a pair trading facility eliminates the leg risk described above without a sacrifice of liquidity. Large Transactions Only Certain large investors may prefer to use pair crossing network 5 rather than pair trading engine 3 to avoid having a trade request broken up into numerous small executions For example, sudden, brief moves in one of the two stocks included in the pair trade request may cause pair trading engine 3 to issue numerous small executions to fill the request While a small investor may welcome capturing these small opportunities, a large investor may find such small executions to be more of a nuisance than a service. Price Setting versus Price Taking Large investors seeking liquidity may prefer to set their price via the pair crossing network 5 Also, other spread investors looking for liquidity can use pair crossing network 5 to monitor and trade with the large investor at the large investor s level While client orders directed to pair trading engine 3 can designate a spread limit, such orders are essentially price-takers as the market reaches the desired level, the orders are executed Moreover, the pair trading engine tranching mechanism creates relatively small orders, allowing institutional flows to move the individual stocks As a result, the small, tranched orders generated by pair trading engine 3 can become overpowered by single-name institutional flows In addition, orders designated solely for pair trading engine 3 and not for pair crossing network 5 are not published to a central quote facility such as by portfolio manager 9 thereby preventing other spread traders from knowing the size and limit of a pair trading engine order. Illiquid Stocks vs Liquid Stocks Spreads that include one or two illiquid stocks are difficult to fill using pair trading engine 3 alone Because illiquid stocks often demonstrate small bid and ask sizes and wide bid-ask spreads, pair trading engine 3 will typically only issue market orders having small quantities subject to user minimums and maximums that presents the client with greater leg risk from mid-trade changes in the bid-ask prices In contrast, orders routed to price crossing network 5 are not confined by liquidity in the market place thereby allowing large crosses between spread traders in illiquid spreads. Accordingly, a system and method for trading pair securities is provided in which the client receives the benefits of having a pair order filled by either pair trading engine 3 pair crossing network 5 or a combination of both. A number of embodiments of the present invention have been described Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention Based on the above description, it will be obvious to one of ordinary skill to implement the system and methods of the present invention in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device Each computer program may be implemented in a high-level procedural or object-oriented programming language, or in assembly or machine language if desired and in any case, the language may be a compiled or interpreted language Suitable processors include, by way of example, both general and special purpose microprocessors Furthermore, alternate embodiments of the invention that implement the system in hardware, firmware or a combination of both hardware and software, as well as distributing modules and or data in a different fashion will be apparent to those skilled in the art and are also within the scope of the invention In addition, it will be obvious to one of ordinary skill to use a conventional database management system such as, by way of non-limiting example, Sybase, Oracle and DB2, as a platform for implementing the present invention Also, network access devices can comprise a personal computer executing an operating system such as Microsoft Windows , Unix , or Apple Mac OS , as well as software applications, such as a JAVA program or a web browser Access devices can also be a terminal device, a palm-type computer, mobile WEB access device or other device that can adhere to a point-to-point or network communication protocol such as the Internet protocol Computers and network access devices can include a processor, RAM and or ROM memory, a display capability, an input device and hard disk or other relatively permanent storage Accordingly, other embodiments are within the scope of the following claims. It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above process, in a described product, and in the construction set forth without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention, which, as a matter of language, might be said to fall therebetween. A method is provided for fulfilling a pair trade request and includes the steps of receiving a plurality of pair trade requests executing a transaction for a first portion of one of the plurality of pair trade requests and matching a second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests. 20.The invention claimed is.1 A computer-implemented method for fulfilling a pair trade request, said pair trade request performed by a financial institution having an order inventory, the method comprising the steps of. receiving a plurality of pair trade requests, comprising at least one pair trade request, wherein said pair trade request comprises a request to trade a first security, a request to trade a second security, and a request to trade said first security and said second security with a minimum spread limit, and wherein said first security and said second security each have a bid price and an ask price. determining the bid bid spread in the market of said first security and said second security. determining the ask ask spread in the market of said first security and said second security. determining that the minimum spread limit of said pair trade request is met by a range of said bid bid spread and said ask ask spread. executing a transaction between a first portion of the trade of said first security in said one pair trade request and at least one non-pair trade request in an external market, provided that the minimum spread limit of said one pair trade request is met by said range of said bid bid spread and said ask ask spread and. executing a transaction between one of a second portion of said trade of said first security in said one pair trade request or a first portion of the trade of said second security in said one pair trade request, and at least one non-pair trade request in said order inventory, provided that the minimum spread limit of said one pair trade request is met by said range of said bid bid spread and said ask ask spread. wherein at least one of said steps is implemented with a computer.2 The method of claim 1 further comprising. executing a transaction between a portion of the trade of said second security in said one pair trade request and at least another non-pair trade request, provided that the minimum spread limit of said one pair trade request is met by said range of said bid bid spread and said ask ask spread. and wherein the steps of executing said first portion of the trade of said first security in said one pair trade request and executing said portion of the trade of said second security in said one pair trade request include the steps of. determining whether the bid price of the first security and the bid price of the second security meet a spread limit. determining an amount of the second security that can be sold based on a bid size associated with the second security. calculating an equivalent amount of said first security that can be bought based on the amount of said second security that can be sold. adjusting said equivalent amount of said first security based on adjustment criteria. calculating a purchase price for said adjusted equivalent amount of said first security based on the spread limit. executing an initiating order to buy said adjusted equivalent amount of said first security at said purchase price and. executing a covering order to sell said amount of the second security.3 The method of claim 2 wherein the step of executing a covering order to sell includes the step of. executing a covering order to sell said amount of the second security at the bid price of the second security.4 The method of claim 2 further comprising the steps of. determining whether the ask price of the first security and the ask price of the second security meet a spread limit. determining an amount of the first security that can be bought based on an offer size associated with the first security. calculating an equivalent amount of said second security that can be sold based on the amount of the second security that can be bought. adjusting said equivalent amount of said second security based on adjustment criteria. calculating a selling price for said adjusted equivalent amount of said second security based on the spread limit. executing an initiating order to sell said adjusted equivalent amount of said second security at said selling price and. executing a covering order to purchase said amount of the first security.5 The method of claim 4 wherein the step of executing a covering order to purchase includes the step of. executing a covering order to purchase said amount of the first security at the ask price of the first security.6 The method of claim 4 wherein said adjustment criteria include a minimum amount and a maximum amount.7 The method of claim 4 wherein the step of executing an initiating order includes the step of. rounding said initiating order to a round lot size.8 The method of claim 1 further comprising. executing a transaction between a portion of the trade of said second security in said one pair trade request and at least another non-pair trade request, provided that the minimum spread limit of said one pair trade request is met by said range of said bid bid spread and said ask ask spread. and wherein the steps of executing said first portion of the trade of said first security in said one pair trade request and executing said portion of the trade of said second security in said one pair trade request includes the steps of. determining whether the ask price of the first security and the ask price of the second security meet a spread limit. determining an amount of the second security that can be bought based on an ask size associated with the second security. calculating an equivalent amount of said first security that can be sold based on the amount of the second security that can be bought. adjusting said equivalent amount of said first security based on adjustment criteria. calculating a selling price for said adjusted equivalent amount of said first security based on the spread limit. executing an initiating order to sell said adjusted equivalent amount of said first security at said selling price and. executing a covering order to purchase said amount of said second security.9 The method of claim 1 wherein at least one of the executing steps include the step of. executing at least a portion of the trade of said first security or said second security in said one pair trade request in a plurality of tranches.10 The method of claim 1 wherein said plurality of pair trade requests comprises another pair trade request, wherein said another pair trade request comprises a request to trade said first security, a request to trade said second security and wherein said one pair trade request has a first spread limit and said another pair trade request has a second spread limit and wherein said method further comprises the steps of. determining that the minimum spread limit of said another pair trade request is met by said range of said bid bid spread and said ask ask spread and. matching another portion of said trade of said first security in said one pair trade request and at least another portion of said trade of said second security in said one pair trade request against said another pair trade request, provided that a range of the minimum spread limit of said one pair trade request and said another pair trade request overlaps with said range of said bid bid spread and said ask ask spread.11 The method of claim 10 wherein said matching step further includes the steps of. determining that a range of said first spread limit and said second spread limit overlaps with a market spread. setting a spread level. calculating prices for the first security and the second security that are within the market spread and based on said spread level and. matching said another portion of said trade of said first security in said one pair trade request and at least said another portion of the trade of said second security in said one pair trade request against said another pair trade request based on said calculated prices.12 The method of claim 11 wherein the step of setting a spread level includes the steps of. calculating a mean between said first spread limit and said second spread limit and. setting said spread level as said mean if said mean is within said market spread.13 The method of claim 12 further including the step of. identifying a spread amount that is closest to said mean and within said market spread and. setting said spread level as said spread amount if said mean is not within said market spread.14 A computer-implemented method for fulfilling a pair trade request, comprising the steps of. receiving a plurality of pair trade requests, comprising at least one pair trade request, wherein said pair trade request comprises a request to trade a first security, a request to trade a second security, and a request to trade said first security and said second security with a minimum spread limit, and wherein said first security and said second security each have a bid price and an ask price. determining the bid bid spread in the market of said first security and said second security. determining the ask ask spread in the market of said first security and said second security. determining that the minimum spread limit of said pair trade request is met by a range of said bid bid spread and said ask ask spread. executing a transaction between a first portion of the trade of said first security in said one pair trade request and at least a first non-pair trade request, provided that the minimum spread limit of said one pair trade request is met by said range of said bid bid spread and said ask ask spread. executing a transaction between a first portion of the trade of said second security in said one pair trade request and at least a second non-pair trade request, provided that the minimum spread limit of said one pair trade request is met by said range of said bid bid spread and said ask ask spread and. repeating the executing steps until the pair trade request is fulfilled. wherein at least one of said steps is implemented with a computer.15 The method of claim 14 wherein at least one of the repeated steps includes the step of. executing a transaction for a portion of the trade of said first security in said one pair trade request in an external market.16 The method of claim 14 wherein at least one of the repeated executing steps is performed by a financial institution having an order inventory and at least said one of said repeated steps includes the step of. executing a transaction for a portion of the trade of said first security in said one pair trade request against said order inventory.17 The method of claim 14 wherein said plurality of pair trade requests comprises another pair trade request, wherein said another pair trade request comprises a request to trade said first security, a request to trade said second security and a request said first security and said second security with a second minimum spread limit, the method further comprising the steps of. determining that the minimum spread limit of said another pair trade request is met by said range of said bid bid spread and said ask ask spread and. matching a second portion of said trade of said first security in said one pair trade request and at least a first portion of the trade of said second security in said one pair trade request against said another pair trade request, provided that a range of the minimum spread limit of said one pair trade request and said another pair trade request overlaps with said range of said bid bid spread and said ask ask spread.18 A computer readable storage medium storing instructions for fulfilling a pair trade request that, when executed by a computer, cause the computer to. receive a plurality of pair trade requests, comprising at least one pair trade request, wherein said pair trade request comprises a request to trade a first security, a request to trade a second security, and a request to trade said first security and said second security with a minimum spread limit, and wherein said first security and said second security each have a bid price and an ask price. determine the bid bid spread in the market of said first security and said second security. determine the ask ask spread in the market of said first security and said second security. determine that the minimum spread limit of said pair trade request is met by a range of said bid bid spread and said ask ask spread. execute a transaction between a first portion of the trade of said first security in said one pair trade request and at least one non-pair trade request in an external market, provided that the minimum spread limit of said one pair trade request is met by said range of said bid bid spread and said ask ask spread and. execute a transaction between one of a second portion of said trade of said first security in said one pair trade request or a first portion of the trade of said second security in said one pair trade request, and at least one non-pair trade request in said order inventory, provided that the minimum spread limit of said one pair trade request is met by said range of said bid bid spread and said ask ask spread.19 A computer readable storage medium storing instructions for fulfilling a pair trade request that, when executed by a computer, cause the computer to. receive a plurality of pair trade requests, comprising at least one pair trade request, wherein said pair trade request comprises a request to trade a first security, a request to trade a second security, and a request to trade said first security and said second security with a minimum spread limit, and wherein said first security and said second security each have a bid price and an ask price. determine the bid bid spread in the market of said first security and said second security. determine the ask ask spread in the market of said first security and said second security. determine that the minimum spread limit of said pair trade request is met by a range of said bid bid spread and said ask ask spread. execute a transaction between a first portion of the trade of said first security in said one pair trade request and at least a first non-pair trade request, provided that the minimum spread limit of said one pair trade request is met by said range of said bid bid spread and said ask ask spread. execute a transaction between a first portion of the trade of said second security in said one pair trade request and at least a second non-pair trade request, provided that the minimum spread limit of said one pair trade request is met by said range of said bid bid spread and said ask ask spread and. repeat the executing steps until the pair trade request is fulfilled.20 The method of claim 19 wherein at least one non-pair trade request is selected from the group consisting of an external market order and an order from an inventory from a financial institution. CROSS-REFERENCE TO RELATED APPLICATIONS. This application is a divisional application of U S patent application Ser No 10 206,549, entitled Pair trading system and method , which was filed on Jul 25, 2002 now U S Pat No 7,412,415, which claims priority to U S provisional patent application Ser No 60 334,163 entitled Method and System for Trading Pairs of Securities that was filed on Nov 29, 2001 The contents of both applications are herein incorporated by reference. The following invention relates to a system and method for trading securities and, in particular, for a system and method of trading securities in pairs. A recognized strategy for trading securities is known as pair-trading Pair-trading is a non-directional investment strategy in which the investor identifies two securities having similar characteristics and the securities are currently trading at a price relationship that is out of their historical trading range The investor exploits the price relationship between the securities by buying the undervalued security while short-selling the overvalued security Because pair-trading is a market-neutral strategy, it is a particularly desirable strategy for investing in volatile markets. One context in which pair trading is useful is where an investor desires to take advantage of an arbitrage opportunity resulting from a merger between two companies For example, Company A has announced a definitive agreement to acquire Company T in which case Company T shareholders will receive 0 5 shares of Company A stock for each share of Company T stock they own The investor desires to capture the spread between the offered consideration 0 5 shares of A and the price of T stock To do this, the investor buys shares in T stock and sells shares of A stock. For instance, if stock T is trading at 28 per share and stock A is trading at 60 per share, then the investor may execute a trade for 200,000 spreads by buying 200,000 shares of T stock and selling 100,000 shares of A stock After the merger takes place, the investor will cover the short position in stock A with the 100,000 shares of A stock the investors receives in exchange of the 200,000 shares the investor held of stock T Thus, by executing the pair trade, the investor locks in a 400,000 profit assuming that the merger goes through The process of executing a pair trade thus includes executing individual trades directed to each leg of the pair trade request An example of a system for executing trades for filling a pair trade request is the Quantex system from ITG of 380 Madison Avenue, New York, N Y 10017.A challenge in implementing a pair trade is to find a counterparty for a particular position an investor desires to establish while minimizing leg risk Typically, a large pair trade is performed off the market as a private transaction negotiated by a financial institution that services large clients For example, if an investor desires to execute a pair trade betting that a proposed merger between two companies will go through, the investor would approach a financial institution seeking an investor that is willing to bet against the merger The financial institution then acts as an intermediary between the two investors in which the investors establish equal and opposite positions in the stock of the proposed merger partners thereby completing the pair trade Thus by matching two pair trade requests so that the transactions associated with each of the pair trade legs are executed simultaneously, neither investor is exposed to leg risk that would otherwise result for the period of time between execution of the first leg and the second leg of the pair trade. There are numerous drawbacks associated with the prevalent pair-trading practice First, pair-trading is typically limited to clients of large financial institutions that have the ability to identify suitable counterparties for a particular pair trade This is especially the case when the pair trade involves a large amount of stock or illiquid stocks in which the only way to execute the trade and minimize leg risk is via an off the market transaction negotiated by a financial institution Also, because a pair-trade is typically negotiated by the parties with a financial institution as an intermediary, the process is often slow and inefficient Furthermore, pair-trading under current practice is generally best suited for large clients seeking to establish large positions thereby providing the financial institutions with the economic incentive to execute the transaction Smaller clients, however, must rely on the markets for executing pair trades, which is unsuitable for illiquid stocks and also results in increased leg risk. Accordingly, it is desirable to provide a system and method for trading securities in pairs. SUMMARY OF THE INVENTION. The present invention is directed to overcoming the drawbacks of the prior art pair trading practices Under the present invention a method is provided for fulfilling a pair trade request and includes the steps of receiving a plurality of pair trade requests executing a transaction for a first portion of one of the plurality of pair trade requests and matching a second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests. In an exemplary embodiment, the method includes the step of executing a transaction for a first portion of one of the plurality of pair trade requests in an external market. In another exemplary embodiment, the method includes the step of executing a transaction for a first portion of one of the plurality of pair trade requests against the order inventory. In yet another exemplary embodiment, the pair trade request includes a first security having a bid price and an ask price and a second security having a bid price and an ask price, and the method includes the steps of determining whether the bid price of the first security and the bid price of the second security meet a spread limit determining an amount of the second security that can be sold based on a bid size associated with the second security calculating an equivalent amount of the first security that can be bought based on the amount of the second security that can be sold adjusting the equivalent amount of the first security based on adjustment criteria calculating a purchase price for the adjusted equivalent amount of the first security based on the spread limit executing an initiating order to buy the adjusted equivalent amount of the first security at the purchase price and executing a covering order to sell the amount of the second security. In still yet another exemplary embodiment, the method includes the step of executing a covering order to sell the amount of the second security at the bid price of the second security. In an exemplary embodiment, the method includes the steps of determining whether the ask price of the first security and the ask price of the second security and or the bid price of the first security and the bid price of the second security meet a spread limit determining an amount of the first security that can be bought based on an offer size associated with the first security calculating an equivalent amount of the second security that can be sold based on the amount of the second security that can be bought adjusting the equivalent amount of the second security based on adjustment criteria calculating a selling price for the adjusted equivalent amount of the second security based on the spread limit executing an initiating order to sell the adjusted equivalent amount of the second security at the selling price and executing a covering order to purchase the amount of the first security. In another exemplary embodiment, the method includes the step of executing a covering order to purchase the amount of the first security at the ask price of the first security. In yet another exemplary embodiment, the adjustment criteria include a minimum amount and a maximum amount. In still yet another exemplary embodiment, the method includes the step of rounding the initiating order to a round lot size. In an exemplary embodiment, the method includes the step of executing a first portion of one of the plurality of pair trade requests in a plurality of tranches. In another exemplary embodiment, the one of the plurality of pair trade requests and the another of the plurality of pair trade requests include a first security and a second security, the one of the plurality of pair trade requests has a first spread limit and the another of said plurality of trade requests has a second spread limit and wherein the method includes the steps of determining that a range of the first spread limit and the second spread limit overlaps with a market spread setting a spread level calculating prices for the first security and the second security that are within the market spread and based on the spread level and matching the second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests based on the calculated prices. In yet another exemplary embodiment, the method includes the steps of calculating a mean between the first spread limit and the second spread limit and setting the spread level as the mean if the mean is within the market spread. In still yet another exemplary embodiment, the method includes the step of identifying a spread amount that is closest to the mean and within the market spread and setting the spread level as the spread amount if the mean is not within the market spread. In an exemplary embodiment, the one of the plurality of pair trade requests and the another of the plurality of pair trade requests include a first security and a second security, the one of the plurality of pair trade requests has a first spread limit, a buy ratio and a sell ratio, the another of the plurality of trade requests has a second spread limit, a buy ratio and a sell ratio and the method includes the steps of determining that the buy ratio and the sell ratio associated with the one of the plurality of trade requests does not equal the buy ratio and the sell ratio of the another of the plurality of trade requests and that an overlap exists between range of the first spread limit and the second spread limit and a market spread determining that market prices exist that are within the overlap determining a mismatch amount in the second security based on a difference between the buy ratio and the sell ratio associated with the one of the plurality of trade requests and the buy ratio and the sell ratio of the another of the plurality of trade requests calculating a cross amount for the first security and the second security selecting a crossing price for the first security and the second security that is within the overlap determining that the mismatch amount is available at the crossing price for the second security matching the second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests based on the calculated prices and executing a transaction for the mismatch amount of the second security at the crossing price for the second security. In another exemplary embodiment, the method includes the step of determining that the mismatch amount is available in an external market at the crossing price for the second security. In yet another exemplary embodiment, the method is performed by a financial institution having order inventory and includes the step of determining that the mismatch amount is available in the order inventory at the crossing price for the second security. In still yet another exemplary embodiment, the one of the plurality of pair trade requests and the another of the plurality of pair trade requests indicate a number of spreads and the method includes the step of matching a second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests if the number of spreads is greater than a minimum number of spreads. In an exemplary embodiment, the method includes the step of receiving a preference for filling at least some of the plurality of trade requests via the step of executing a transaction for a first portion of one of the plurality of pair trade requests, described above. In another exemplary embodiment, the method includes the step of receiving a preference for filling at least some of the plurality of trade requests via the step of matching a second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests, described above. Under the present invention, a method for fulfilling a pair trade request is provided and includes the steps of receiving a plurality of pair trade requests and matching at least a portion of one of the plurality of pair trade requests against another of the plurality of pair trade requests. Under the present invention, a system for fulfilling a pair trade request is provided, the system receiving a plurality of pair trade requests and includes a pair trading engine for executing a transaction for a first portion of one of the plurality of pair trade requests The system also includes a pair crossing network for matching a second portion of said one of the plurality of pair trade requests against another of the plurality of pair trade requests. In an exemplary embodiment, the system includes a link to external markets and wherein the pair trading engine executes the transaction for the first portion of one of the plurality of pair trade requests in the external markets. In another exemplary embodiment, the system includes a financial institution having an order inventory and wherein the pair trading engine executes the transaction for the first portion of one of the plurality of pair trade requests against the order inventory. In yet another exemplary embodiment, the pair trade request includes a first security having a bid price and an ask price and a second security having a bid price and an ask price, and wherein the pair trading engine determines whether the bid price of the first security and the bid price of the second security meet a spread limit determines an amount of the second security that can be sold based on a bid size associated with the second security calculates an equivalent amount of the first security that can be bought based on the amount of the second security that can be sold adjusts the equivalent amount of the first security based on adjustment criteria calculates a purchase price for the adjusted equivalent amount of the first security based on the spread limit executes an initiating order to buy said adjusted equivalent amount of the first security at the purchase price and executes a covering order to sell the amount of the second security. In still yet another exemplary embodiment, the pair trading engine executes a covering order to sell the amount of the second security at the bid price of the second security. In an exemplary embodiment, the pair trading engine determines whether the ask price of the first security and the ask price of the second security meet a spread limit determines an amount of the first security that can be bought based on an offer size associated with the first security calculates an equivalent amount of the second security that can be sold based on the amount of the second security that can be bought adjusts said equivalent amount of the second security based on adjustment criteria calculates a selling price for the adjusted equivalent amount of the second security based on the spread limit executes an initiating order to sell the adjusted equivalent amount of the second security at the selling price and executes a covering order to purchase the amount of the first security. In another exemplary embodiment, the pair trading engine executes a covering order to purchase the amount of the first security at the ask price of the first security. In yet another exemplary embodiment, the pair trading engine rounds the initiating order to a round lot size. In still yet another exemplary embodiment, the pair trading engine executes a first portion of one of the plurality of pair trade requests in a plurality of tranches. In an exemplary embodiment, the one of the plurality of pair trade requests and the another of the plurality of pair trade requests include a first security and a second security, the one of the plurality of pair trade requests has a first spread limit and the another of the plurality of trade requests has a second spread limit and wherein the pair crossing network determines that a range of the first spread limit and the second spread limit overlaps with a market spread sets a spread level calculates prices for the first security and the second security that are within the market spread and based on the spread level and matches the second portion of said one of the plurality of pair trade requests against another of the plurality of pair trade requests based on the calculated prices. In another exemplary embodiment, the pair crossing network calculates a mean between the first spread limit and the second spread limit and sets the spread level as the mean if the mean is within the market spread. In yet another exemplary embodiment, the pair crossing network identifies a spread amount that is closest to the mean and within the market spread and sets the spread level as the spread amount if the mean is not within the market spread. In still yet another exemplary embodiment, the one of said plurality of pair trade requests and the another of the plurality of pair trade requests include a first security and a second security, the one of the plurality of pair trade requests has a first spread limit, a buy ratio and a sell ratio, the another of the plurality of trade requests has a second spread limit, a buy ratio and a sell ratio and wherein the pair crossing network determines that the buy ratio and the sell ratio associated with the one of the plurality of trade requests does not equal the buy ratio and the sell ratio of the another of the plurality of trade requests and that an overlap exists between range of the first spread limit and the second spread limit and a market spread determines that market prices exist that are within the overlap determines a mismatch amount in the second security based on a difference between the buy ratio and the sell ratio associated with the one of the plurality of trade requests and the buy ratio and the sell ratio of the another of the plurality of trade requests calculates a cross amount for the first security and the second security selects a crossing price for the first security and the second security that is within said overlap determines that the mismatch amount is available at the crossing price for the second security matches the second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests based on the calculated prices and executes a transaction for the mismatch amount of the second security at the crossing price for the second security. In an exemplary embodiment, the pair crossing network determines that the mismatch amount is available in an external market at the crossing price for the second security. In another exemplary embodiment, the pair crossing network determines that the mismatch amount is available in the order inventory at the crossing price for the second security. In yet another exemplary embodiment, the one of said plurality of pair trade requests and the another of the plurality of pair trade requests indicate a number of spreads and wherein the pair crossing network matches a second portion of the one of the plurality of pair trade requests against another of the plurality of pair trade requests if the number of spreads is greater than a minimum number of spreads. In still yet another exemplary embodiment, the plurality of pair trade requests include at least some pair trade requests indicating a preference for execution via said pair crossing network, and the system further includes a portfolio manager in communications with the pair crossing network, the portfolio manager receiving the plurality of pair trade requests and routing the at least some pair trade requests to the pair crossing network according to the preference. In an exemplary embodiment, the system includes a pair trading engine for executing at least some of the plurality of pair trade requests, further includes a portfolio manager in communications with the pair trading engine and wherein the plurality of pair trade requests include at least some pair trade requests indicating a preference for execution via the pair trading engine, the portfolio manager receiving the plurality of pair trade requests and routing the at least some of the plurality of trade requests to the pair trading engine according to the preference. Under the present invention, a system for fulfilling a pair trade request is provided, wherein the system receives a plurality of pair trade requests and includes a pair crossing network for matching at least one of the plurality of pair trade requests against another of the plurality of pair trade requests. Accordingly, a method and a system are provided for trading pair securities. The invention accordingly comprises the features of construction, combination of elements and arrangement of parts that will be exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims Other features and advantages of the invention will be apparent from the description, the drawings and the claims. DESCRIPTION OF THE DRAWINGS. For a fuller understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawings, in which. FIG 1 is a block diagram of a system for trading securities in pairs according to the present invention. FIG 2 is a flowchart of the steps a pair trading engine included in the system of FIG 1 applies to fill a pair trade request. FIG 3 is a flowchart of the steps a pair crossing network included in the system of FIG 1 applies to fill a pair trade request. FIG 4 is a flowchart of a process by which the pair crossing network of the system of FIG 1 fills imperfectly matched orders and. FIG 5 is a graph for identifying the market prices for two securities that meet the required spread limits. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS. Referring now to FIG 1 there is shown a block diagram of a system 1 for trading securities in pairs according to the present invention System 1 receives pair trade requests from clients operating client access devices 7 and attempts to fill the pair trade requests according to the parameters associated with the particular pair trade request System 1 includes two different subsystems for filling pair trade requests a pair trading engine 3 and a pair crossing network 5 As will be described below, pair trading engine 3 receives a pair trade request and attempts to fill in whole or in part the trade request by executing the appropriate trades in an external market 13 that may include, by way of non-limiting example, the New York Stock Exchange, the NASDAQ or any other financial market Pair trading engine 3 may also fill in whole or in part a pair trade request by executing a transaction against order inventory 11 of non-pair trade requests controlled by the financial institution that is operating system 1 In addition, pair trading engine may also fill in whole or in part a pair trade request by forwarding the trade request to pair crossing network 5 for matching with other pair trade requests. Likewise, pair crossing network 5 receives a pair trade request and fulfills in whole or in part the request by matching it against another pair trade request received by pair crossing network 5 by matching the request against inventory 11 controlled by the financial institution and or by forwarding the trade request to pair trading engine 3 for execution in external markets 13.System 1 also includes a portfolio manager 9 that may be, for example, a software program executing on a computer system that receives the pair trade requests from client access device 7 and presents the trade request to either pair trading engine 3 pair crossing network 5 or both, depending on the trade parameters set by the client Also, the client may query portfolio manager 9 regarding the status of any pair trade request the client has presented to system 1.In operation, system 1 may fulfill a pair trade request either using pair trading engine 3 or pair crossing network 5 or a combination of the two For example, a pair trade request received by system 1 may be completely filled by pair trading engine 3 as follows. Assume a case where XYZ is taking over ABC and is offering 0 575 shares of XYZ for each ABC share and investor Arb wants to invest in the price difference between ABC stock and XYZ stock To take advantage of the price difference, Arb wants to lock in the difference between the value offered 0 575 XYZ stock and the value of ABC stock by buying ABC stock and selling XYZ stock subject to the condition that ABC 0 575 XYZ 1 19 i e Arb desires to capture a 1 19 difference between XYZ s takeover offer and ABC s share price. In order to fill this pair trade, Arb presents a pair trade request to portfolio manager 9 using client access device 7 The pair trade request typically includes a number of parameters that define the pair trade and that also may be used by portfolio manager 9 in determining how the pair trade request is to be filled Arb typically indicates in the trade request the number of spreads the Arb desires to invest in and also provides a minimum and maximum share amount that he is willing to trade per tranche. For example, Arb may indicate a desire to invest in 100,000 spreads and may only wish to trade the spread 3,000-8,000 shares at a time Arb generally sets this tranche size range based on the liquidity and volatility of ABC stock and XYZ stock Arb may set a larger minimum tranche size if ABC stock and XYZ stock are fairly liquid stocks because higher liquidity increases the likelihood that a larger tranche size will be executed Arb may set a lower maximum tranche size if XYZ stock and ABC stock are volatile stocks so as to limit the leg risk associated with executing a pair trade. Yet another pair trade parameter Arb provides is the spread limit in the above case 1 19 which is the amount Arb desires to capture in the trade Arb does not have to provide, however, the discrete prices at which trades for ABC and XYZ stock are to be executed as these prices are calculated by pair trading engine 3 and or pair crossing network 5 , as will be described below. Referring now to FIG 2 there is shown a flowchart describing the steps pair trading engine 3 applies to fill a pair trade request The flowchart in FIG 2 is based on the above example and the market data listed in Table 1 below. Initially, in Step 201 pair trading engine 3 determines whether the bid bid prices or ask ask prices of ABC and XYZ stock, respectively, meet the spread limit requirement of the particular pair trade request In this case the bid bid spread is 1 4375 122 50 0 575 69 and the ask ask spread is 1 1969 122 625 0 575 69 3125 so that each spread is less than the spread limit of 1 19, as is required for this particular trade Once it is determined that either the bid bid spread or the ask ask spread meets the spread limit, then in Step 202 it is determined as is indicated in Table 1 how much XYZ stock can be sold at the bid and how much ABC stock can be bought at the ask In an exemplary embodiment, the client may specify whether the bid bid spread, the ask ask spread or either the bid bid or the ask ask spread must exceed the indicated spread limit for a transaction to proceed If neither the bid bid spread nor the ask ask spread meets the spread limit, the process waits a period of time for example 0 10 seconds and returns to Step 201 to again test whether the bid bid spread or the ask ask spread meets the spread limit. Next, in step 203 an equivalent amount of stock that can be sent into the market i e bought sold in the market is calculated for a spread based on the bid bid price spread and or the ask ask price spread that meets the spread limit In this example, if a maximum of 10,000 shares of XYZ stock can be sold into the market i e the XYZ bid size then, based on the ABC XYZ ratio of 1 0 575 in this case , a total of 17,391 10,000 0 575 shares of ABC stock are to be bought in order to execute a balanced pair trade Likewise, if a maximum of 1,500 shares of ABC stock can be bought in the market i e the ABC ask size , then, based on the ABC XYZ ratio of 1 0 575 in this case , a total of 863 1500 0 575 shares of XYZ stock are to be sold in order to execute a balanced pair trade. Next, in Step 204 the pair trade share amounts calculated in Step 203 are adjusted to conform to the wave maximum and minimum parameters i e the maximum minimum tranche size included in the pair trade request as well as market round lot limits In the above example, the amount of ABC shares to be bought that was calculated based on the XYZ bid size i e 17,391 is first rounded to an even lot size i e 17,400 and then reduced to the maximum tranche size of 8000 Also, the amount of XYZ shares to be offered that was calculated based on the ABC ask size i e 863 is first rounded to an even lot size i e 900 and then increased to 1,700 shares to meet the minimum tranche size of 3000 3000 0 575 1777 In an exemplary embodiment the minimum and maximum tranche size is scaled by the particular ratio for example, in the above case, the tranche sizes for XYZ stock is scaled by 0 575 In another embodiment, the maximum minimum tranche size is used for each security in the pair trade request without scaling In yet another exemplary embodiment, the pair trade request includes a separate maximum minimum tranche for each security. Once the share amounts for the pair trade are calculated, in Step 205 the share prices that are needed to meet the spread limit of the pair trade request are calculated For example, for a pair trade based on the bid bid price spread, in order to meet the spread limit of 1 19 credit, the price at which ABC stock is to be bid should be no greater than 69 2475 122 50 0 575 1 19 a share Likewise, for a pair trade based on the ask ask price spread, in order to meet the spread limit, the price at which XYZ stock is to be offered should be greater than or equal to 122 6130 69 3125 1 19 0 575 a share. Next, once the pair trade share amounts and share prices have been calculated, in Step 206 pair trading engine 3 sends initiating orders to external markets 13 in order to fill the pair trade request The initiating orders may include an initiating order for executing a pair trade based on the bid bid spread in this case a bid for 8,000 shares of ABC stock at 69 2475 and or an initiating order for executing a pair trade based on the ask ask spread in this case an offer of 1,700 shares of XYZ stock at 122 6130.Finally, as the initiating orders sent to external markets 13 in Step 207 get filled, pair trading engine 3 automatically sends into the market the covering side of the pair trade So, for example, as the initiating order of buying 8,000 shares of ABC stock at 69 2475 gets filled, pair trading engine 3 sends an order to external markets 13 to sell 4,600 8,000 0 575 shares of XYZ stock at 122 50.In an exemplary embodiment, the client s pair trade request includes threshold amounts that indicate the amount of variance in stock price and or share amount the client is willing to absorb For example, if in the process of covering the initiating order the price of XYZ stock dips to 122 49 in which case the spread limit of the pair trade would drop to 1 18 , then pair trading engine 3 would still sell XYZ stock at the price of 122 49 if the 0 01 difference was within the threshold amount included in the pair trade request Similarly, the pair trade request may include threshold amounts for any other pair trade parameter, including by way of non-limiting example, the number of spreads to be purchased and the tranche sizes If, however, a particular threshold amount indicated by the client is exceeded for any given pair trade parameter, then pair trading engine 3 would attempt to cancel the initiating order and or the covering order that may be possible if the orders have not yet reached the market or have not yet been filled In such a case, pair trading engine 3 would then repeat the above analysis for determining suitable initiating and cover orders. To fill a pair trade request, pair trading engine 3 executes trades utilizing the method described above Typically, pair trading engine 3 tranches a pair trade request and trades piece-meal in external markets 13 In certain cases, however, it may be difficult to fill a trade request by executing several transactions in external markets 13 either because the pair trade request is for a very large number of spreads or includes stocks that are illiquid in which cases pair trading engine 3 may be ineffective in filling the pair trade request Also, in certain situations, a client wishing to remain anonymous may indicate in the pair trade request a preference that no orders be sent to external markets 13 In these circumstances, portfolio manager 9 may route the particular pair trade request to pair crossing network 5.Referring now to FIG 3 there is shown a flowchart illustrating the steps pair crossing network 5 applies to fill a pair trade request The flowchart in FIG 3 is based on the above example and the market data listed in Table 2 below. Spread Limit as defined by 0 575 XYZ ABC. Continuing the previous example, assume the pair trade request issued by Arb for 100,000 spreads was half-filled by pair trading engine 3 Also, assume that system 1 receives a pair trade request from Antiarb that indicates a desire to sell 30,000 shares of ABC and buy 17,200 shares a ratio of 1 0 575 and also indicates a spread limit of 1 30 i e ABC 0 575XYZ 1 30 In this case Arb and Antiarb s orders are complimentary in the primary order elements securities, ratios and buy versus sell Also, Antiarb is willing to pay 0 11 per spread more than Arb is demanding from the marketplace Based on these parameters, there is an opportunity for Arb s and Antiarb s trade requests to be filled via pair crossing network 5.If Antiarb s pair trade request was marked for trading by pair trading engine 3 then portfolio manager 9 sends Antiarb s order to pair trading engine 3 for execution Pair trading engine 3 then sends the parameters of Antiarb s trade request, as well as all orders waiting for execution in pair trading engine 3 to pair crossing network 5 Pair crossing network 5 will recognize as described above that there is a crossing opportunity between Arb s order and Antiarb s order In this case, pair crossing network 5 then directs pair trading engine 3 to suspend the execution of Antiarb s order in the amount that can be crossed by pair crossing network 5 30,000 spreads in this case In addition, pair trading engine 3 routes a cross amount of 30,000 spreads from Arb s order to pair crossing network 5 for crossing against Antiarb s order At this point, the pair crossing network 5 crosses the Antiarb order against a portion of Arb s order, as follows. Assume the prevailing market conditions at the time of the cross are as shown in Table 3 Furthermore, Table 3 indicates the XYZ Ratio-Adjusted Value for both the bid and ask prices based on the conversion ratio of 1 0 575 Based on the XYZ Ratio-Adjusted Values, a Bid Ask Spread Range i e the spread provided for a cross between the bid price of ABC stock and the XYZ Ratio-Adjusted ask price of 1 3863 is calculated and an Ask Bid Spread Range i e the spread provided for a cross between the ask price of ABC stock and the XYZ Ratio-Adjusted bid price of 1 05 is calculated. To perform the cross, in Step 301 pair crossing network 5 first determines whether the range of spread limits associated with Arb s and Antiarb s trade requests i e 1 30- 1 19 coincides with the range of the prevailing market spread 1 3863- 1 05 In this example, the range of spread limits does coincide with the prevailing market spread because at least a portion of the spread limit range overlaps with a portion of the market spread Thus, a cross can occur. Next, in Step 302 pair crossing network 5 calculates the mean of Arb s and Antiarb s spread order limit which is 1 30 1 19 2 1 245 and determines whether the mean is within the range of the market spread i e 1 3863- 1 05 If it is, then in Step 303 pair crossing network 5 calculates the prices at which to cross The prices must be within the current markets for ABC stock and XYZ stock, and satisfy market uptick requirements for short sales , and provide a spread that is equal to the spread level calculated above For example, with the inside market for ABC stock at 70 00-70 25 and the inside market for XYZ stock at 124 00-124 15, a cross price of 70 11 for ABC stock and 124 096 for XYZ stock provides the spread of 1 2452 thereby meeting the requirement of both Arb s and Antiarb s trade request Finally, in Step 304 pair crossing network 5 crosses 30,000 shares of ABC stock at 70 11 with Arb buying and Antiarb selling and 17,200 shares of XYZ at 124 096 with Arb selling and Antiarb buying. If it is determined in Step 302 that the mean of Arb s and Antiarb s spread order limits does not fall within the range of the market spread, then in Step 305 the spread closest to the mean of the two spread limits that is also within the market spread is calculated For example, if the market spread is 1 3863- 1 28, then the mean of the two spread limits 1 245 is not within the market spread In such a case, 1 28 is selected as the spread level that is closest to the mean and within the market spread In an exemplary embodiment, the spread level at which Arb and Antiarb cross can be determined in any other suitable manner as long as the spread level is within the market spread and within the range of spread limits indicated in the pair trade requests. Once the spread level is determined, the method proceeds to Step 303 in which pair crossing network 5 calculates prices to cross at that are within the current markets for ABC stock and XYZ stock and that meet the calculated spread level In the case where the calculated spread level is 1 28, the cross will occur at a price of 70 08 for ABC stock and 124 1043 for XYZ stock Finally, the method proceeds to Step 304 in which pair crossing network 5 performs the cross between Arb and Antiarb. Once a pair trade request is filled or partially filled , the transaction details are reported to portfolio manager 9 and made available to the client operating client access device 7.In the previous example, pair crossing network 5 crosses orders in which both Arb and Antiarb desire to trade the same pair of securities in the same ratio In an exemplary embodiment, pair crossing network 5 executes a cross between two pair trade requests that are not perfectly matched. For example, assume that pair crossing network 5 receives the pair trade requests as shown in Table 4 Note that these two pair trade requests are imperfectly matched because each trade request uses a different ratio between ABC and XYZ stock. Arb s Spread Limit is defined by 0 575 XYZ ABC Antiarb s Spread Limit is defined by 0 6 XYZ ABC. Also, assume the market in ABC and XYZ stocks at the time the pair trade requests are received by pair crossing network 5 is as described in Table 5 below. Arb s XYZ Ratio-Adjusted Value. Arb s Dollar Range. AntiArb s XYZ Ratio-Adjusted Value. AntiArb s Dollar Range. Referring now to FIG 4 there is shown a flowchart illustrating a process by which pair crossing network 5 fills these imperfectly matched order First, in Step 401 pair crossing network 5 determines whether Arb s buy security equals Antiarb s sell security and whether Arb s sell security equals Antiarb s buy security If both conditions are not met, then a cross between the two orders cannot occur If the two conditions are met, then in Step 402 it is determined whether Arb s buy ratio equals Antiarb s sell ratio and whether Arb s sell ratio equals Antiarb s buy ratio If these ratios are the same, then pair crossing network 5 proceeds to cross the two orders as described in the example above Note that for a cross to occur at this stage does not require the ratios themselves to match but rather that the ratios of the ratios match for e g a ratio of 2 3 matches a ratio of 0 667 1.If, however, the two ratios are not equal as in this case where Arb s sell ratio does not equal Antiarb s buy ratio , then in Step 403 pair crossing network determines whether there is an overlap between Arb s and Antiarb s spread limit that also falls within the bid ask market for ABC and XYZ stock To make such a determination, pair crossing network 5 calculates whether there are market prices for both ABC and XYZ stock that satisfy the following inequalities L 1 Ratio A ABC Ratio B XYZ and 1 L 2 Ratio C ABC Ratio D XYZ 2 Where L 1 is Arb s spread limit of 1 19 credit, L 2 is Antiarb s spread limit of 4 40 debit, RatioA is Arb s buy ratio of 1 1, RatioB is Arb s sell ratio of 1 0 575, RatioC is Antiarb s sell ratio of 1 1 and RatioD is Antiarb s buy ratio of 1 0 6.Referring now to FIG 5 there is shown a graph 51 that depicts market prices for ABC and XYZ stock that meet the spread limits of Arb and Antiarb In graph 51 the x-axis represents the prices for XYZ stock while the y-axis represents the prices for ABC stock Graph 51 includes a shaded area 53 that is the universe of market prices for ABC and XYZ stock that could satisfy the spread trade involving those stocks Also included in graph 53 is a spread limit line L 1 inequality 1 , above that represents the spread limit associated with Arb and a spread limit line L 2 inequality 2 , above that represents the spread limit associated with Antiarb Thus, the solution set of market prices that satisfies inequalities 1 and 2 is the portion of dark shared area 53 that falls between spread limit line L 1 and spread limit line L 2 In this example, a cross at a share price for ABC of 70 14 and a share price of 124 15 for XYZ stock meets the investor s spread limits and falls within the market prices for ABC and XYZ stock. If it is determined that no share prices for both ABC and XYZ stock satisfy Arb s and Antiarb s spread limits, then no cross can occur If such share prices do exist, then in Step 404 it is determined which of the investors desires to transact in fewer shares of ABC stock and a mismatch in share amounts caused by the differing ratios is determined In our example, Antiarb desires to sell fewer ABC shares than Arb desires to buy 30,000 vs 50,000 Then, in Step 405 pair crossing network 5 determines the number of XYZ shares that can be crossed between Arb and Antiarb based on the maximum amount of ABC shares that can be crossed 30,000 in this example Based on the Antiarb ABC order quantity of 30,000 shares, the maximum number of XYZ shares that Arb will cross with Antiarb is.30 000 Arb XYZ Ratio Arb ABC Ratio 30 000 0 575 1 17 300 17 250 rounded to an even lotsize. While the maximum quantity of XYZ shares that Arb will cross is 17,300, Antiarb s trade request indicates a desire to cross 18,000 shares To overcome this imbalance, in Step 406 pair crossing network 5 is in communications with external markets 13 for determining whether the excess 700 XYZ shares needed to satisfy Antiarb s trade request can be transacted for in external markets 13 In an exemplary embodiment, pair crossing network 5 makes this determination by issuing a query to pair trading engine 3 as to whether 700 shares of XYZ stock can be bought in external markets 13 Because, as indicated in Table 5, 3,000 shares of XYZ stock are offered at 124 15, pair trading engine 3 responds to pair crossing network 5 that the 700 shares needed to balance the cross between Arb and Antiarb are available from external markets 13 at 124 15.Next, in Step 407 pair crossing network calculates the cross prices that are necessary such that Arb and Antiarb achieve their respective spread limits while also incorporating the excess 700 shares of XYZ stock that must be purchased from external markets 13 at 124 15 to satisfy Antiarb s trade request An example of such cross prices that meet these criteria is a price of 70 14 for ABC stock and a price of 124 15 for XYZ stock. Once the cross prices are calculated, in Step 408 pair crossing network 5 crosses 30,000 shares of ABC stock and 17,300 shares of XYZ stock between Arb and Antiarb and also buys 700 shares of XYZ stock at 124 15 in external markets 13 on behalf of Antiarb Thus, both Arb and Antiarb s pair trade requests are satisfied. Alternatively, the entire 18,000 shares of XYZ stock may be crossed thereby fully satisfying Antiarb s trade request In such a case, the ratio mismatch is addressed by Arb purchasing an additional 1200 700 0 575 rounded to a lotsize shares of ABC stock from external market 13 or from firm inventory 11.Once the trade is completed, the details of the transaction are provided to portfolio manager 9 to report the transaction details to the investors. In an exemplary embodiment, a pair order or portion thereof may be filled against an internal inventory 11 of trade requests maintained by the financial institution operating system 1 For example, in the previous example in which an excess of 700 shares of XYZ stock needs to be purchased in order for a match i e cross between Arb and Antiarb s trade requests to occur, instead of determining whether the 700 shares are available in external markets 3 pair crossing network 5 examines firm inventory 11 to determine whether the shares are available at the required price Likewise, in cases where pair trading engine 3 desires to execute a pair trade based on orders to be sent to external markets 13 pair trading engine 3 may first determine whether the order can be filled, in whole or in part, using trade requests pending in firm inventory 11 Generally, the advantages of filling an order using pending trade requests in firm inventory 11 is that execution is faster, transaction costs are lower and leg risk is minimized. In another exemplary embodiment, a client s pair trade request may also include a minimum number of spreads that can be traded in pair crossing network 5 Also, pair crossing network 5 may be designed to require a minimum share amount for a cross to occur A minimum number of spreads that can be traded may be provided in order to reduce the distractions and booking costs associated with numerous smaller trades that may exceed the benefits of a de minimis fill. In another exemplary embodiment, portfolio manager 9 publishes the inside cross market for any pair that a client has selected for crossing in pair crossing network 5 In still another exemplary embodiment, the client has the option for each pair trade selected for crossing in pair crossing network 5 to designate that the order should be reflected in the published inside cross market This inside cross market consists of the tightest spread bid and offer and corresponding bid size and offer size from all client pair orders pending in pair crossing network 5 In this way, a client can assess the likelihood and timing of a pair trade request being filled by pair crossing network 5 Also, by publishing the client s spread interest, others seeking liquidity can trade at the client s level. In an exemplary embodiment, the client can designate each pair order designated for pair trading engine 3 and or pair crossing network 5 for Broker Negotiation If Broker Negotiation is designated, the client s broker-dealer sales representative is notified of the client s spread order thereby prompting the broker-dealer to solicit a complementary, agency order from another client The client may also designate each pair order for Broker Facilitation in which case the client allows the broker-dealer to act principally to fill the client s order. In summary, the advantages to a client of using pair trading engine 3 is that pair trading engine 3 allows the client to trade a spread order while limiting leg risk or the risk of missing a targeted spread level This is accomplished by breaking the total order into tranches of sizes proportionate to the market, subject to user minimums and maximums, that can be traded in external markets 13 or against firm inventory 11 Orders executed via pair trading engine 3 however, are typically of a lower traded volume because trading is constrained to the liquidity available in the market In contrast, trades executed via pair crossing network 5 are not constrained by market liquidity and do not have to be tranched to minimize leg risk In particular, the benefits of filling a pair trade request via pair crossing network 5 are as follows. Elimination of Leg Risk Pair crossing network 5 potentially provides a deeper well of liquidity because the trades are brokered, as a spread, directly between spread investors via a central clearing facility Moreover, the introduction and use of a pair trading facility eliminates the leg risk described above without a sacrifice of liquidity. Large Transactions Only Certain large investors may prefer to use pair crossing network 5 rather than pair trading engine 3 to avoid having a trade request broken up into numerous small executions For example, sudden, brief moves in one of the two stocks included in the pair trade request may cause pair trading engine 3 to issue numerous small executions to fill the request While a small investor may welcome capturing these small opportunities, a large investor may find such small executions to be more of a nuisance than a service. Price Setting versus Price Taking Large investors seeking liquidity may prefer to set their price via the pair crossing network 5 Also, other spread investors looking for liquidity can use pair crossing network 5 to monitor and trade with the large investor at the large investor s level While client orders directed to pair trading engine 3 can designate a spread limit, such orders are essentially price-takers as the market reaches the desired level, the orders are executed Moreover, the pair trading engine tranching mechanism creates relatively small orders, allowing institutional flows to move the individual stocks As a result, the small, tranched orders generated by pair trading engine 3 can become overpowered by single-name institutional flows In addition, orders designated solely for pair trading engine 3 and not for pair crossing network 5 are not published to a central quote facility such as by portfolio manager 9 thereby preventing other spread traders from knowing the size and limit of a pair trading engine order. Illiquid Stocks vs Liquid Stocks Spreads that include one or two illiquid stocks are difficult to fill using pair trading engine 3 alone Because illiquid stocks often demonstrate small bid and ask sizes and wide bid-ask spreads, pair trading engine 3 will typically only issue market orders having small quantities subject to user minimums and maximums that presents the client with greater leg risk from mid-trade changes in the bid-ask prices In contrast, orders routed to price crossing network 5 are not confined by liquidity in the market place thereby allowing large crosses between spread traders in illiquid spreads. Accordingly, a system and method for trading pair securities is provided in which the client receives the benefits of having a pair order filled by either pair trading engine 3 pair crossing network 5 or a combination of both. A number of embodiments of the present invention have been described Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention Based on the above description, it will be obvious to one of ordinary skill to implement the system and methods of the present invention in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device Each computer program may be implemented in a high-level procedural or object-oriented programming language, or in assembly or machine language if desired and in any case, the language may be a compiled or interpreted language Suitable processors include, by way of example, both general and special purpose microprocessors Furthermore, alternate embodiments of the invention that implement the system in hardware, firmware or a combination of both hardware and software, as well as distributing modules and or data in a different fashion will be apparent to those skilled in the art and are also within the scope of the invention In addition, it will be obvious to one of ordinary skill to use a conventional database management system such as, by way of non-limiting example, Sybase, Oracle and DB2, as a platform for implementing the present invention Also, network access devices can comprise a personal computer executing an operating system such as Microsoft Windows , Unix , or Apple Mac OS , as well as software applications, such as a JAVA program or a web browser Access devices can also be a terminal device, a palm-type computer, mobile WEB access device or other device that can adhere to a point-to-point or network communication protocol such as the Internet protocol Computers and network access devices can include a processor, RAM and or ROM memory, a display capability, an input device and hard disk or other relatively permanent storage Accordingly, other embodiments are within the scope of the following claims. It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above process, in a described product, and in the construction set forth without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention, which, as a matter of language, might be said to fall therebetween.
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