In music, a prolation 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. Accompanying voices may enter either simultaneously or successively. If voices extend the rhythmic values of the leader, a procedure known as augmentation, the resulting canon can be called an augmentation canon or canon by augmentation or sloth canon. Conversely, if they reduce the note values in diminution, it can be called a diminution canon or canon by diminution. Prolation canons are among the most difficult canons to write, and are relatively rare in the repertory, though they are most common in the early Renaissance and from the 20th century to the present. Examples of prolation canons from the Renaissance include Le Ray Au Soleyl by Johannes Ciconia ; the entire Missa prolationum by Johannes Ockeghem, in which each separate section of the mass explores a different prolation ; the Agnus Dei from the Missa L'homme armé super voces musicales by Josquin des Prez ; and the Agnus Dei from the Missa L'homme armé by Pierre de la Rue. Johann Sebastian Bach is known for his Canon a 4 per Augmentationem et Diminutionem, the last in a set of 14 canons written as an appendix to the Goldberg Variations. In the 20th century, one such canon is the Cantus in MemoriamBenjamin Britten by Arvo Pärt. Additionally, Larry Polansky has written numerous four-voice prolation canons whose melodies are permutations of a limited number of elements, and Mark Alburger, in Immortality from San RafaelNews, directly maps a new melody into the framework of the aforementioned Josquin. A particularly striking example of prolation canon occurs twice in the opening movement of Shostakovich's Symphony No. 15, first in the strings and later in the woodwind at . A more recent example of a prolation canon in contemporary music is rindenmotette by Austrian composerKlaus Lang.
In this example, the first 12 bars of the Agnus Dei II of the earlier of the two masses Josquin wrote based on the L'homme armé tune, each voice sings the same music, but at different speeds. The top voice is barred in 3/4 meter for clarity. The slowest voice is the one in the middle. The lowest voice sings the same music at twice the speed of the slowest, and the highest voice sings the same music at three times the speed of the slowest. In the original score, only one part is given: a notation over the single line of music indicates the three prolations to be used, and a second notation over the line indicates where each voice should end if sung correctly.
