organum

Organum is a term used in medieval music to describe a style of polyphony in which one or more voices accompany a plainchant, providing harmony through additional melodic lines. The chant serves as the basis, or cantus firmus, and a voice (or voices) moves in a variety of intervals to create concords with the chant. In early organum, dating from the 9th century, the added voice typically moved in fixed parallel intervals to the chant, producing parallel organum. This practice is described in medieval treatises such as Musica enchiriadis and Scolica enchiriadis, which introduced the concept of organal voices sung above or below the chant.

By the 12th and 13th centuries, the Notre Dame school expanded organum into more elaborate textures. Léonin and his successor Pérotin compiled three- and four-voice pieces, often designated as organum duplum, organum triplum, and organum quadruplum. In florid organum, the higher voices moved in elaborate melismatic lines while the lower voice held a slow, sustained cantus firmus (tenor). In discant organum, voices moved more uniformly with note-for-note or near-simultaneous motion, creating a distinct rhythmic and contrapuntal texture.

Notations became increasingly precise, with rhythmic modes guiding performance and the use of ligatures indicating patterns of long and short notes. The organum tradition laid the groundwork for later medieval polyphony and influenced the broader development of Western music, gradually evolving into more varied rhythmic and modal systems in the Ars Nova period.