Imitation
One voice repeats, after a delay, the motive just stated by another. It can imitate at the unison, at the octave, at the fifth, or by transforming the motive: by inversion (intervals flipped), augmentation (values doubled), diminution, retrograde.
Canon
This is strict, continuous imitation: the second voice repeats the first exactly for the whole duration.
- Canon at the unison / round: everyone sings the same melody, entering in turn (“Frère Jacques”).
- Canon at the octave or at the fifth.
- Canon over an ostinato bass: the voices in canon above a repeating bass.
How to write one
- Choose a short progression (e.g. two bars: C – G – Am – F) and write the first phrase over it.
- The second voice will enter over the same progression two bars later: write the continuation of the first voice so that it sounds good against its own entrance.
- Use chord tones on strong beats and complementary rhythms (when one voice moves, the other holds).
Fugue (a brief mention)
Subject in the tonic, answer at the dominant, countersubject accompanying the answer, sequential episodes, stretti (closely spaced entries).
In pop and production
Call and response between vocal and synth, ad-libs that pick up the lead's phrase, backing vocals that enter staggered in the bridge, rhythmic synced delays that create automatic “canons.”
Real-world example
Pachelbel's Canon in D is a canon for three violins over a two-bar ostinato bass. In the “operatic” section of “Bohemian Rhapsody” (Queen, 1975) the voices answer each other by imitation.