Composer. The first major Western composer known to us by name, he was a leading light of the Notre Dame or Parisian School. He was hailed as a master of organum, the earliest form of polyphonic (multi-voiced) music, in which a second melodic line was added to traditional plainchant. Léonin compiled and contributed to the original version of the "Magnus Liber Organi" ("Great Book of Organum", c. 1175), the most important single collection of Medieval vocal works. It is a huge body of two-part settings of Mass and Vespers sections to celebrate major feasts of the liturgical year. Many also believe he created the first rational systems for rhythmic practice and notation, as the "rhythmic modes" were a defining feature of the Notre Dame School. Virtually nothing is known of Léonin's life. He was probably a Frenchman living in Paris as a priest and teacher at Notre Dame Cathedral during its initial phases of construction (1163 to 1210). His experiments with the second melodic line showed growing complexity and were said to have reflected his desire to compose something commensurate with the vast Gothic architecture and acoustics of the new church. The "Magnus Liber" he assembled contained the active repertory of all the early Notre Dame composers, most unnamed. Its original version apparently preserved over 100 of Léonin's compositions, but the book was later substantially revised by his great successor Pérotin. Now it is difficult to determine which works are by Léonin and his followers, apart from 13 responses for the Canonical Hours and 32 mass responses. The most substantial piece attributed to him is the two-voice "Viderunt omnes". But the testimony of late 13th Century English music theorist Anonymous IV confirms that Léonin's reputation was still held in awe many years after his death. A seven-volume modern edition of the "Magnus Liber" is in print, and selections have been performed and recorded by Early Music ensembles.