The Magnus Liber or Magnus liber organi (English translation: Great Book of Organum), written in Latin, was a repertory of medieval music known as organum. This collection of organum survives today in three major manuscripts. This repertoire was in use by the Notre-Dame school composers working in Paris around the end of the twelfth and beginning of the thirteenth centuries, though it is well agreed upon by scholars that Leonin contributed a bulk of the organum in the repertoire. This large body of repertoire is known from references to a "magnum volumen" by Johannes de Garlandia and to a "Magnus liber organi de graduali et antiphonario pro servitio divino" by the English music theorist known as Anonymous IV. Today it is known only from later manuscripts containing compositions named in An