Canon (from the Latin meaning 'rule,' is the strictest form of counterpoint. After the initial statement of a melody in one "voice," all subsequent "voices" must imitate that melody exactly, note for note, or with only minimal adjustments. The melody must be composed so that it sounds correct when played against itself. The imitations may begin on the same pitch, or on another pitch (in which case all the notes will have to be transposed to maintain the integrity of the melody). That voice in a canon which first enters with the melody to be imitated is called the antecedent (or dux, 'leader'), and any imitating voice is known as the consequent (or comes, 'companion').
Canons are usually part of larger works; perhaps the most renowned collection of canons is contained in J. S. Bach's Musical Offering. "Row, Row, Your Boat" is a familiar example of a simple canon; ; so is "Frére Jacques," which appears in the third movement of Mahler's First Symphony.
Types of canon
A canon at the octave is one in which the voices (human or instrumental) are at that pitch-interval from one another. A canon at the fifth, or at any other interval, is similarly explained.
A canon for two voices is called a canon two in one (and similarly with canon three in one, etc.). A canon four in two is a double canon, i.e. one in which two voices are carrying on one canon while two others are engaged on another.
A canon by augmentation has the imitating voices in longer notes than the one that they are imitating. Canon by diminution is the reverse of this.
A perpetual canon or infinite canon is a canon so arranged that each voice, having arrived at the end, can begin again, and so indefinitely. The converse is finite canon.
A strict canon is that in which the intervals of the imitating voice are exactly the same as those of the voice imitated (i.e. as regards their quality of major, minor, etc.).
In a free canon the intervals remain the same numerically, but not necessarily as to quality (e.g. a major third may become a minor third).
A crab canon, is a contrapuntal piece in which one part is identical to another, but backwards.