A strictly ordered texture in which polyphony is derived from a single line by imitation of itself at fixed intervals of time and pitch. In other words, all the canonic parts are the same, but they overlap each other. The term canon originally referred to the verbal, symbolic, or cryptic rule by which the imitations are formed.
The follower must be created from the leader by being either an exact replication of the rhythms and intervals of the leader, or a transformation such as those listed in "types of canons" (below). In the fourteenth century many canons were written in Italy under the name caccia, and occasionally French chansons of that period used canon technique.Types of canons
The most rigid and ingenious forms of canon are not strictly concerned with pattern but also with content. Canons are classified by various traits: the number of voices, the interval at which each successive voice is transposed in relation to the preceding voice, whether voices are inverse, retrograde, or retrograde-inverse;
How voices in a canon are named
Although, for clarity, this article uses leader and follower(s) to denote the leading voice in a canon and those that imitate it, musicological literature also uses the traditional Latin terms Dux and Comes for "leader" and "follower", respectively.
Number of voices
A canon of two voices may be called a canon in two, similarly a canon of x voices would be called a canon in x.
Another standard designation is "Canon: Two in One", which means two voices in one canon. "Canon: Four in Two" means four voices with two simultaneous canons. "Canon: Six in Three" means six voices with three simultaneous canons, and so on.
Interval
An interval canon imitates the leader at any interval other than the octave or unison (e.g. If the follower imitates the precise interval quality of the leader, then it is called an exact canon; if the follower imitates the interval number (but not the quality), it is called a diatonic canon.
Inverse
An inverted canon (also called canon in contrary motion) moves the follower in contrary motion to the leader. A sub-order of canon in contrary motion, "mirror," maintains the precise quality of each interval.
Retrograde
In a crab canon, also known as cancrizans, the follower accompanies the leader backward (in retrograde).
Mensuration and tempo canons
In a mensuration canon (also known as a prolation canon, or a proportional canon), the follower imitates the leader by some rhythmic proportion. The follower may double the rhythmic values of the leader (augmentation or sloth canon) or it may cut the rhythmic proportions in half (diminution canon). The cancrizans, and often the mensuration canon, take exception to the rule that the follower must start later than the leader.
Technically, mensuration canons are among the most difficult to write. Johannes Ockeghem wrote an entire mass (the Missa Prolationum) in which each section is a mensuration canon, and all at different speeds and entry intervals.
Other types of canons
The most familiar of the canons might be the perpetual/infinite canon (in latin: canon perpetuus). As (each voice of) the canon arrives at its end it can begin again, in a Perpetuum mobile fashion;
Additional types include the spiral canon, accompanied canon, and double or triple canon.
Puzzle canon
A Puzzle canon can be any of the above types, but only one voice is notated, and it is up to the performer to find out which rule applies to the canon. Machaut's rondeau Ma fin est mon commencement et mon commencement est ma fin (My end is my beginning and my beginning is my end) is a crab canon with a third voice which is a musical palindrome.
Elaborate use of canon technique
Josquin Desprez, Missa L'homme armé super voces musicales, Agnus Dei 2: One voice writte with the words 'ex una voce tres' (three voice parts out of one), a mensuration canon in three voices in different tempos. Josquin Desprez, Missa L'homme armé sexti toni, Agnus Dei 2: two simultaneous canons in the four upper voices, and at the same time a crab canon in the two lower voices.Contemporary canons
The most popular canons heard today are from the Baroque period, such as Johann Pachelbel's Canon in D (Pachelbel's Canon) or every third variation in Bach's Goldberg Variations. Henryk Górecki's Third Symphony begins with an extensive eight voice canon in the strings. Steve Reich uses a process he calls phasing which is a canon with variable distance between the voices.
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