during the late 12th and early 13th centuries, an important group of composers and singers working under the patronage of the great Cathedral of Notre-Dame in Paris. The Notre-Dame school is important to the history of music because it produced the earliest repertory of polyphonic (multipart) music to gain international prestige and circulation. Its four major forms are organum (q.v.), a setting (for two to four voice parts) of a chant melody in which the chant is sung in sustained notes beneath the florid counterpart of the upper voice(s); clausula (q.v.), actually a section within an organum composition corresponding to a melismatic (many notes per syllable) section of the chant and characterized by a decisive acceleration of pace in the voice having the chant; conductus (q.v.), a processional composition in chordal style and not derived from any preexistent chant; and motet (q.v.), similar to the clausula, from which it evidently evolved, but with the addition of new texts, often secular, in the upper parts.

The composers of the Notre-Dame school are all anonymous except for two, Léonin (q.v.), or Leoninus (late 12th century), and Pérotin (q.v.), or Perotinus (flourished c. 1200), both of whom are mentioned in a 13th-century treatise by an anonymous Englishman studying in Paris. According to the treatise, Léonin excelled in the composition of organa and, in fact, composed the Magnus liber organi (“Great Book of Organa”), which contains a series of two-part organa for the entire liturgical year. Pérotin, the apparent successor to Léonin, is cited for his three- and four-voice organa, as well as his “substitute clausulae,” newly composed clausulae intended for insertion within the older organa.