Composers of organum at the cathedral of Notre-Dame de Paris in the late 12th and early 13th centuries.
Léonin (с 1135–1201?) is credited with composing two-voice florid organum characterized by a rhythmically patterned "melisma" (a series of notes sung on one syllable) added to each sustained note of the plainchant (see Gregorian chant ). He may have devised the rhythmic notation (ligatures) that made this possible or at least codified the important system of rhythmic mode s. His younger contemporary Pérotin (fl. с 1200) is said to have edited, extended, and added parts to Léonin's Magnus liber organi ("Great Book of Organum") and created the first three-and four-voice textures known in world music. See also Ars Antiqua .