A contrapuntal procedure where the melody in a primary voice (the leader) is strictly and completely imitated, after a delay, by a second voice (the follower), the continuous combination of both voices producing a polyphonic texture.
Most of us are familiar with the idea of canon from a specialized form of canon known as a round. "Row Your Boat" and "Three Blind Mice" are often sung as canons, becoming rounds when the first voice starts the song again in a continuous, circular form (also known as a circular canon). The key concept of the canon demonstrated here is that each voice sings the same melody but starts at different times. Somehow, the combination works to form a coherent, musical whole.
The topic of canon is in fact much more elaborate and complicated that this, and well beyond the scope of this definition. While canon (a word with Greek origins meaning "law" or "rule") is a much more strict procedure than fugue, its rules allow for much more flexibility than the simple, familiar round suggests. Canons can involve more that two voices and may also be accompanied by a part that is not bound by the primary rules. Imitation can start at different times and on different pitches. The follower's part, though strictly derived from the leader, may enjoy transformations such as you find in fugue and invention: inversion or contrary motion, augmentation, diminution, and retrograde (backwards). You can see that "strict" imitation has a variety of possibilities. Canon is an important form and procedure based on counterpoint and, as such, is related to fugue.
From the perspective of fugue itself, canon is important as a basic procedure of imitative counterpoint often used as part of a fugue. While canon is not fugue, many fugues contain portions of canonic imitation. There are two clear examples: stretto and canonic episodes.
Recall that stretto occurs when two or more subject or countersubject entries overlap. Here is a diagram of stretto taken from Bach's Well-Tempered Clavier, Book 2, No.5 in D major:
Notice that, unlike the exposition, in stretto, a new subject entry begins before the previous entry has completed. Since all subject entries are essentially strict imitations of the same, primary subject, stretto is actually a miniature canon. This particular diagram indicates two sections of stretto separated by an episode, hence, two little canons embedded within the fugue.
Canonic imitation is also frequently used in fugue episodes where it creates compelling complexity and texture, a section of musical coherence and personality independent of formal subject entries. Most often, the canonic imitation is based on a sequence. Here is an example from Bach's Well-Tempered Clavier, Book 1, No. 24 in B minor:
This is referred to as a canonic sequence, or, a sequential canon. Often, such an episode is repeated (with some modification) later in the fugue where, because of its recurrence, it carries a greater depth. These canonic sequences are frequently embedded within the full texture of the fugue and therefore accompanied by additional voices using free counterpoint.
Examples of canonic imitation abound in the fugue literature of both Bach and Shostakovich. One of the most poignant examples is used here for illustration. In Bach's Well-Tempered Clavier, Book 1, No. 24 in B minor, a highly chromatic and "difficult" subject contrasts strongly with the lovely canonic episodes, recurring multiple times in a manner that is soothing, almost, one might say, compassionate.
Bach wrote many extraordinary and complex canons, the most famous coming from any one of his monumental works: the Goldberg Variations, Canons on the Goldberg Ground, Canonic Variations on Vom Himmel Hoch, the Art of Fugue and a Musical Offering. Some of these works also contain some fascinating and important fugues. But like fugue, the canon persists as a musical form or procedure, particularly as a section within a larger movement. Haydn's string quartet Op. 76 No. 2 in D minor uses a strict circular canon for a minuet.
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