. When Maurice de Sully became bishop of Paris in 1160, he launched the construction of the cathedral of Notre-Dame, which was largely completed by 1250. An abundance of polyphonic music for the Masses and Offices of major feasts of the liturgical year was created during this period to enhance the ceremonies at the cathedral. This musical repertory developed in two phases. The first involved the compilation of the Magnus liber organi by Léonin, named by the music theorist Anonymous 4 as the earliest member of this school. This new polyphony, the most advanced music then devised, introduced modal rhythm into segments of the polyphony known as discant clausulae, which were composed in note-against-note style to the plainchant melismas, in contrast to the lengthy and mellifluous lines improvised or composed to the syllabic sections of the chant. The innovation of a musical notation for these clausulae proved a turning point in western music. The clausulae grew in importance, as composers explored ways to expand, extend, and notate increasingly complex rhythms; numerous independent clausulae apparently intended to substitute for the original sections in the Magnus liber organi survive in the Notre-Dame sources W1 and F. Some of these clausulae may have been a factor in the abbreviatio (“abbreviation” or “revision”) of the Magnus liber organi that Anonymous 4 attributed to Pérotin, whom he called optimus discantor.
The second phase consisted of revisions and additions introduced by Pérotin, whom Anonymous 4 named as Léonin’s successor. It is probable that the notation if not much of the music of the Magnus liber organi itself was revised and that new organum was added over time, so that the three extant versions of the Magnus liber organi reflect three layers, all of which apparently postdate the abbreviatio attributed to Pérotin. The version in the Florence manuscript (F) may be closer to the liturgical practice of Notre-Dame and is both larger and later than that in W1.
Craig Wright has proposed, therefore, that the version in F may be closer to Léonin’s Magnus liber organi and that dissemination from Paris was selective rather than additive, as had previously been thought. Anonymous 4 also implied a distinction between the two phases that is valuable in understanding their musical styles. Léonin’s polyphony is predominately two-part organum, whereas Pérotin’s is generally discant in two, three, or four parts. This liturgical polyphony quickly spread throughout Europe, notably in England and Spain, and manuscript fragments in other northern regions indicate an even wider diffusion. As it spread, however, the spirit of innovation that had imbued its formation was increasingly swept up in the vernacular tide that began in the early 13th century, culminating in the French-texted motet that dominated the second half of the century.
According to Anonymous 4, Pérotin also composed four- and three-part polyphony for the Graduals and Alle-luias of Masses for Christmas, Easter, and the Nativity of the Virgin Mary, and several conductus. The notational and rhythmic innovations of Pérotin and ‘his colleagues in the cathedral were probably the basis of the earliest systematic theoretical writings on the rhythmic modes, those attributed to Johannes de Garlandia, which formed the “key” by which Friedrich Ludwig in the early decades of the 20th century unlocked an understanding of the obscure notation of the Notre-Dame organa and clausulae.
