Prolation canon
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A prolation canon is a type of musical canon. It is sometimes also called a mensuration canon.
In a prolation canon, each voice not only sings or plays the same music, but at different speeds (or prolations, a metrical term which dates to the medieval and Renaissance eras). Voices may either enter successively or simultaneously. Prolation canons are among the most difficult to write, and are relatively rare in the repertory.
Examples of prolation canons include the entire Missa Prolationum (Johannes Ockeghem), in which each separate section of the mass explores a different prolation (or different gap between entries and relative speed of each voice), and the Cantus In Memoriam Benjamin Britten by Arvo Pärt (1976), which was used in Michael Moore's 2004 film Fahrenheit 9/11.