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In music, a prolation canon (also called a mensuration canon or proportional canon) is a type of canon, a musical composition wherein the main melody is accompanied by one or more imitations of that melody in other voices. Not only do the voices sing or play the same melody, they do so at different speeds (or prolations, a mensuration term that dates to the medieval and Renaissance eras). Accompanying voices may enter either simultaneously or successively.
If voices extend the rhythmic values of the leader (for example, by doubling all note values), a procedure known as augmentation, the resulting canon can be called an augmentation canon or canon by augmentation (canon per augmentationem) or sloth canon (recalling the slow movement of the sloth). Conversely, if they reduce the note values in diminution, it can be called a diminution canon or canon by diminution (canon per diminutionem).