History
The canon has its origins in Italy and France and was originally called caccia, the latin word for 'chase'. The Old French canon, which meant 'leaned', was taken from the Greek kanon for a rule or law, which eventually came to mean 'an accepted rule' in English. The most rigid and ingenius forms of canon are not strictly concerned with pattern but also with content. During the period of the Netherland School (1450-1500), canon as a contrapuntal art form received its greatest development, while the Roman School gave it its most complete application.
Types of canons
Canons are classified by various traits: the number of voices, the interval at which each succesive voice is transposed in relation to the preceding voice, whether voices are inverse, retrograde, or retrograde-inverse; the temporal distance between each voice, whether the intervals of the second voice are exactly those of the original or if they are adjusted to fit the diatonic scale, and the tempo of succesive voices. However, canons may use more than one of the above methods.
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. This terminology may be used in combination with a similar terminology for the interval between each voice, different from the terminology in the following paragraph. Interval
An interval canon imitates the leader at any interval other than the octave or unison (e.g. canon at the second, fifth, seventh, etc.). If the follower imitates the precice 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.Contrapuntal derivations
The follower may be a contrapuntal derivation of the leader.Inverse
An inverted canon (also called canon in contrary motion) moves the follower in contrary motion to the leader. Where the leader would go down a fifth, the follower goes up, and vice versa. 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 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). Phasing involves the application of modulating rhythmic proportions according to a sliding scale. The cancrizans, and often the mensuration canon, take exception to the rule that the follower must start later than the leader.Other types of canons
The most familiar of the canons might be the perpetual/infinite canon. As the canon arrives at the end it can begin again; e.g. "Three Blind Mice". Additional types include the spiral canon, accompanied canon, and double or triple canon.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. What may be George Rochberg's best known work, his String Quartet No. 6, is a set of variations on the Pachelbel Canon in D. Conlon Nancarrow has written complex tempo or mensural canons, mostly for the player piano as they are extremely difficult to play and has influenced many younger composers. Henryk Górecki's Third Symphony begins with an extensive eight voice canon on the strings. Steve Reich discovered a process he calls phasing which is a canon with variable distance between the voices. Many popular recording artists have found success by sampling portions of famous canons in their compositions.