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In late medieval Western music, a clausula was a newly composed polyphonic section for two or more voices sung in discant style ("note against note") over a cantus firmus. Clausulae eventually became used as substitutes for passages of original plainchant. They occur as melismatic figures based on a single word or syllable within an organum (a composition where one or more voices have been added to a plainchant melody to create polyphony). The text of a clausula differs from that of the plainchant melody underneath it. Each clausula is clearly delineated by a final cadence.
Clausulae emerged from the compositional practices of the Notre Dame school in Paris c. 1160-1250 (during the stylistic period known as ars antiqua), especially those of the composers Léonin and Pérotin. Rather than write entirely new music, the preference was to take existing music, that is, plainchant melodies, and develop or improve upon them. Pérotin's clausulae make use of the rhythmic modes, whose strict metrical feet necessitated that voices change notes together (discantus). This was in contrast to the earlier practice of one voice moving in a free rhythm above a "tenor" voice (Latin tenere: "to hold") sustaining the long notes of a cantus firmus. The tenor line was often repeated to allow for expansion of the clausula; this was the origin of the technique known as isorhythm.
Hundreds of clausulae in two, three and four parts were incorporated into the Magnus Liber Organi of Léonin and Pérotin. Others were arranged in liturgical order within appropriate manuscripts so that they could be easily introduced into a particular organum setting or piece of plainchant. As they were notated separately, it was possible for them to be expanded and developed further, and they eventually became standalone pieces which could be sung at certain points in the liturgy. The composition of clausulae died out in the mid-13th century as they were replaced by motets as the main platform for the development of new compositional techniques.