Anonymous IV in the context of Franco of Cologne

The Notre-Dame school or the Notre-Dame school of polyphony refers to the group of composers working at or near the Notre-Dame Cathedral in Paris from about 1160 to 1250, along with the music they produced.

The only composers whose names have come down to us from this time are Léonin and Pérotin. Both were mentioned by an anonymous English student, known as Anonymous IV, who was either working or studying at Notre-Dame later in the 13th century. In addition to naming the two composers as "the best composers of organum," and specifying that they compiled the big book of organum known as the Magnus Liber Organi, he provides a few tantalizing bits of information on the music and the principles involved in its composition. Pérotin is the first composer of organum quadruplum—four-voice polyphony—at least the first composer whose music has survived, since complete survivals of notated music from this time are scarce.

Léonin (also Leoninus, Leonius, Leo; fl. 1163–1190) was the first known significant composer of polyphonic organum. He was probably French, probably lived and worked in Paris at the Notre-Dame Cathedral and was the earliest member of the Notre Dame school of polyphony and the ars antiqua style who is known by name, thanks to the writer known as Anonymous IV. Though no further identification is certain, the name "Leoninus" and its Latin diminutive Leo have the French equivalents Léonin/Léo.

Pérotin (fl. c. 1200) was a composer associated with the Notre Dame school of polyphony in Paris and the broader ars antiqua musical style of high medieval music. He is credited with developing the polyphonic practices of his predecessor Léonin, with the introduction of three and four-part harmonies.

Other than a brief mention by music theorist Johannes de Garlandia in his De Mensurabili Musica, virtually all information on Pérotin's life comes from Anonymous IV, a pseudonymous English student who probably studied in Paris. Anonymous IV names seven titles from a Magnus Liber—including Viderunt omnes, Sederunt principes and Alleluia Nativitas—that have been identified with surviving works and gives him the title Magister Perotinus (Pérotinus the Master), meaning he was licensed to teach. It is assumed that Perotinus was French and named Pérotin, a diminutive of Peter, but attempts to match him with persons in contemporary documents remain speculative.

The Magnus Liber or Magnus liber organi (English translation: Great Book of Organum), written in Latin, is a repertory of medieval music known as organum. This collection of organum survives today in three major manuscripts. This repertoire was in use by the Notre-Dame school composers working in Paris around the end of the twelfth and beginning of the thirteenth centuries, though it is well agreed upon by scholars that Leonin contributed a bulk of the organum in the repertoire. This large body of repertoire is known from references to a "magnum volumen" by Johannes de Garlandia and to a "Magnus liber organi de graduali et antiphonario pro servitio divino" by the English music theorist known as Anonymous IV. Today it is known only from later manuscripts containing compositions named in Anonymous IV's description. The Magnus Liber is regarded as one of the earliest collections of polyphony.