Léonin, or Magister Léoninus, is identified through a thirteenth century English source, Anonymous 4, as the composer and compiler of the Magnus Liber organi de gradali et antiphonario pro servitio divino. This massive work originally consisted of musical settings for the entire church year, feast days and Saints days, with two-voice polyphonic responses for each scripture reading. The Magnus liber was compiled about 1170, and was utilized first by the choir of the Cathedral of Notre Dame, still under construction at the time the book appeared. This music is known in four major manuscripts; Wölfenbuttel 677 (W1) and 1099 (W2), Codex Florenz, and Codex Madrid, the last named containing a condensed version of the Magnus liber. W1, though chronologically the "youngest" of these late thirteeth century manuscripts, is believed to contain the music which is closest to Léonin's original concept. Anonymous 4 refers to Léonin as "optimus organista," and in his work he employed a two-part polyphonic texture which Léonin termed Organum Duplum; the tenor was the "principal voice" (vox principalis), generally intoning long syllables drawn from plainchant, and an "organizing voice" (vox organalis) which added freely rhythmic melismata up above. Léonin usually alternated these passages with discant sections, where the tenor and organizer would move more or less together in a note-per-syllable pattern. Sections of chant not set by Léonin would be sung out by the choir in unison, and sometimes the choir would read along with the tenor during the discants. But the organizing and principal parts were sung by expert vocalists who were placed at the center of the choir. As construction of the huge sanctuary progressed at Notre Dame, the desirability of added "triplum" and even "quadruplum" parts to the Magnus liber became an issue; the text of the work began to change under other hands, most famously those of Léonin's alleged pupil Perotin. It is apparent in the Florenz, W2, and Madrid sources that long sections of the organalis seen in W1 have been wiped away in favor of new, and more elaborate material. That's why it's difficult to separate out specific pieces within the Magnus liber as being safely within the camp of Léonin and his school, apart from 13 responses intended for the Canonical Hours and 32 responses to specific masses. In the original work, more than a hundred pieces would have been preserved in Organum Duplum. Some scholars have suggested Léonin as a possible author of some of the three-voice organa, though Anonymous 4 states otherwise. Léonin was also known as a poet, and is said to have written a rendering of the first eight books of the Holy Bible in verse. In the late twentieth century, a scholar proposed that Léonin was likely the author of a salacious book concerning carnal behavior among the clergy which appeared in the 1180s. While Perotin's settings of the Magnus liber are more frequently performed, Léonin's achievement in compiling the book is on a par within Western sacred music with the Lutheran hymnal or Bach's cantata cycle. Although Léonin died more than eight centuries ago, scholars are yet engaged on an enthusiastic search for Léonin, including details about his life and his status as one of the first great composers in western music history.