Organum

Organum is the broad term for early medieval polyphony built on a Gregorian chant. The simplest form (parallel organum, 9th-10th centuries) doubled the chant a fixed interval below; by the 11th century the added voice began to move more freely against the chant. In the great Parisian organum of the 12th and 13th centuries (Leoninus, Perotinus and the Notre-Dame School) the chant became a held tenor of enormously sustained pitches, with one to three faster upper voices moving above it. Harmonies favor open fifths and octaves, and the music is sung a cappella by male voices.

The earliest written description is in the anonymous 'Musica enchiriadis' of around 900. Practice evolved through the 11th-century 'Winchester Troper' and the school of Saint-Martial de Limoges before reaching its peak at Notre-Dame in Paris under Leoninus (c. 1175) and Perotinus (c. 1200). The rise of the University of Paris and the development of rhythmic notation made coordinated polyphony writable for the first time. By the late 13th century the held-tenor style had given way to the motet and the new ars antiqua repertoire.

Listen to how the tenor voice sits almost as a drone, while the upper voices ornament above. The intervals at structural points are open fifths and octaves rather than the warmer thirds and sixths of later music. Recordings in resonant stone spaces — or with long artificial reverb — restore the architectural character the music was written for.

Perotinus's 'Viderunt omnes' (c. 1198) is the canonical entry. For the older Leonine layer try 'Alleluia Pascha nostrum' or the two-voice settings in the 'Magnus Liber Organi.'

Despite the name, organum has nothing to do with the organ as an instrument — the term refers to the technique of adding voices to chant. Some scholars connect the word to the Greek 'organon' (instrument) in the sense of an articulated musical structure.