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A clausula (plural clausulae) is a polyphonic composition performed as a musical alternative to the original plainchant passage that it is intended to replace.

Clausulae came into being as a result of the composition practice of musicians in the Notre Dame school period, between the 11th and 13th centuries. Rather than composing new pieces, musicians concerned themselves with taking older compositions and developing them. These older compositions were the original plainchant melodies, only some of which were specifically set, and they were in an organum style that used monophony and polyphony as its principal means of contrast. The new compositions, specifically called ‘clausul sive puncta’, allowed the musicians of the time to be more inventive and to develop compositional techniques. They made use of the new modal rhythms that replaced monophony and polyphony as the principal means of contrast. Clausulae often involved melismatic polyphony being used against a tenor line that was repeated to allow for the expansion, and this lies at the roots of isorhythm.

Hundreds of clausulae in two, three and four parts were incorporated into the Magnus Liber and others into liturgical order with appropriate manuscripts and assembled into small collections so that they could be easily introduced into an organum setting or into a piece of plainchant. As they were notated separately, it was possible for them to be expanded and developed further, resulting in them becoming pieces in their own right which could be sung at certain points in the liturgy as independent compositions. The composition of clausulae died out in the mid 13th century as they were replaced by motets, which some scholars believe to have evolved from clausulae, as the main platform for the development of new compositional techniques.

• The New Grove Dictionary of Music and Musicians
• The Oxford Companion to Music