Term used, following the late 13th-century theorist known as Anonymous IV, for the most important and widely-influential collection of two-voice polyphony from the early Notre Dame period. Originally compiledc 1170, from the period Notre Dame Cathedral was under construction (1163-82), the Magnus Liber is said to have been the work of Léonin with subsequent revisions by Pérotin. The three main extant versions reveal that the original corpus comprised settings of the solo sections of responsories for Vespers and Mass for certain major feasts of the liturgical year, as celebrated at Notre Dame, in the earlier, sustained-note style of organum. Before 1200 these were reworked in the new styles brought about by the adoption of rhythmic modes, formulated in Parisc 1180, and compositions selected from other churches of the city or diocese were introduced. The repertory underwent several stages of development before the discant style, in which both voices are governed by rhythmic modes, was reached. While Pérotin was undoubtedly responsible for much of the reworking of the Magnus Liber, he cannot be credited as sole creator of the modal system, the new discant style or the resulting items for three or four voices, composed for the most important festal occasions; it is more likely that such far-reaching developments were evolved by a school of composers based at Notre Dame and consolidated by him.
