At the end of the twelfth century, as the walls of the Cathedral of Notre Dame were slowly raised above the surrounding buildings of Paris, a school of composers associated with this Cathedral were in the process of fashioning a new type of musical structure to elevate the divine liturgy performed there. Surviving musical manuscripts from a number of European centers (Santiago de Compostella, St. Martial in Limoges, Winchester Cathedral) from the eleventh and twelfth centuries preserve early experiments in liturgical polyphony, adding to the prescribed plainchant sung in worship an extra ornamental voice for a more glorious service upon major feast days. But the center of Paris, home of an increasingly vigorous bourgeois class, and a renowned University, would host the revolutionary new music known as Notre Dame Organum.
Unfortunately, almost nothing is known of the two major composers of this school, Leonin and Pérotin. The barest of outlines survives in a thirteenth century document containing the notes of an anonymous English university student. He records that a Magister Leoninus, a great composer, produced an entire Magnus liber organi (Great Book of Organum) for use in celebrations of the liturgy; scholars believe this compilation took place between 1160 and 1180. The student (known as Anonymous IV) goes on to note that Magister Perotinus, an even better composer of "discant," revised the work of the earlier Master, adding to it many pieces of his own; presumably, this took place either in the 1180s and 1190s, or early in the following century. From the account of Anonymous IV, and from other contemporary historical records, specific music in surviving manuscripts is attributed to Pérotin the Great. For instance, an 1198 liturgical ordinance by the Bishop of Paris relating New Years' eve observances prohibits a popular ritual known as the Feast of Fools, and stipulates instead of these vulgar revels a solemn service including specific pieces of chant set in Organum.
Possibly these exact pieces by Pérotin -- Viderunt omnes and Sederunt principes, for the liturgy of Christmas and St. Stephen (Dec. 26), respectively -- represent the earliest music surviving in Europe set for four voices. A fragment of Gregorian chant, usually sung by a soloist, is in this style of Organum presented in a single voice, with very extended rhythmic values. Upon this tonal foundation Pérotin erects an interlocking series of upper voices, which vocalize melismatically upon the extended chant syllables. These upper voices adopt vigorous and sequential dance-rhythms made possible by a new theory of notation known as the rhythmic modes. In a complete performance, sections called Clausulae of this highly ornamented polyphony would alternate with passages of unison plainchant, and with shorter chordal sections in "discant" style. In this second style, also used to set Latin liturgical poetry, the voices move jointly according to the rhythmic patterns. Pérotin may have written or revised in excess of 150 discant-style liturgical pieces, most for two voices.