100% that is known all about him comes from either a writings of a late student at the cathedral called Anonymous IV, an Englishman world health organization left a treatise in theory & who mentions Leonin when a composer of the Magnus Liber, the "great book" of organum. Lot of the Magnus Liber is devoted to clausulae—melismatic portions of Gregorian chant which were extracted into separate pieces, sustaining the original value greatly slowed down, & provided with a convenient-moving upper a share. Léonin was besides probably a number one composer to have a rhythmic modes, and even as well to invent the notation for the children (according to W.G. Waite, writing in 1954: "It was Léonin's incomparable achievement to introduce a rational system of rhythm into polyphonic music for the first time, and, equally important, to create a method of notation expressive of this rhythm.")

A Magnus Liber was arranged for liturgical use. Based on data from Anonymous IV, "Magister Leoninus (Léonin) was the finest composer of organum; he wrote the great book (Magnus Liber) for the gradual and antiphoner for the sacred service." All of the Magnus Liber is for 2 voices, although little is known astir actual performance practice: them voices were does'nt necessarily soloists.

Based on data from Anonymous IV, Léonin's function was greatly improved & expanded per late composer Pérotin. Look at as well Medieval music.