What Is a Canon in Music? One Melody Chasing Itself
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A canon is a piece in which one melody is played against a delayed copy of itself. Every voice sings the same tune, entering at different times, and the composer's whole task is writing a line that still makes harmonic sense when overlaid on itself, which is a constraint so tight that it produces some of the most ingenious writing in music.
How it works
The defining requirement is strict imitation: a leading voice states the melody and one or more following voices repeat it exactly after a fixed delay. Because all voices are playing the same material at different points, the composer is effectively writing a melody that must harmonise with itself at a specific time offset, which drastically limits what each note can be. A round, of which Frere Jacques and Row Your Boat are the familiar examples, is the simplest case: a canon at the unison that returns to the beginning indefinitely, so the whole thing can be sung in a circle with no ending. The distinction from a fugue matters, since a fugue introduces its subject in each voice in turn and then develops freely, while a canon maintains the strict imitation throughout, which makes the canon the more constrained form and the fugue the more flexible one.
The variations
Composers extended the idea in directions that increase the constraint considerably:
- •Canon at an interval, where the following voice enters a fifth or an octave above or below rather than at the same pitch
- •Canon by inversion, where the imitating voice turns the melody upside down, so every rising interval becomes a falling one
- •Canon by retrograde, also called crab canon, where the follower plays the melody backwards, which means the piece works read from either end
- •Mensuration or proportional canon, where the follower plays the same melody at a different speed, typically half or double
- •Mirror canons combining inversion and retrograde simultaneously
- •Puzzle or riddle canons, notated as a single line with a cryptic instruction indicating how the second voice should derive its part, which the performer had to solve
- •The perpetual canon, constructed so it modulates and returns, including one by Bach that rises a whole tone each time and arrives back after six repetitions
Where the form peaked
Canonic writing was a serious demonstration of craft from the medieval period onward, with the thirteenth-century English round Sumer Is Icumen In being among the earliest surviving examples and a remarkably sophisticated one. Renaissance composers used canon extensively within masses and motets, sometimes concealing it so that a listener hears a smooth texture without noticing the structure. The form reached its most extreme development with Bach, whose Goldberg Variations place a canon at every third variation, each at a successively wider interval from the unison to the ninth, all while the same bass line underpins every variation. His Musical Offering, built on a theme supplied by Frederick the Great, includes a set of puzzle canons and the crab canon that reads identically forwards and backwards. The Art of Fugue contains further examples. That body of work is frequently described as mathematical, and the more accurate description is that it treats an extremely tight constraint as an opportunity rather than an obstacle.
Where it went
Canon persisted after the baroque period as a technique rather than a genre. Classical and romantic composers embedded canonic passages within larger works for particular effect, since strict imitation produces a distinctive sense of inevitability and pursuit, which is why it appears at moments of tension and of resolution. Rounds remained a staple of communal and educational singing precisely because they allow untrained groups to produce harmony without anyone learning a separate part. The twentieth century revived strict canon in new contexts: serial composers used canonic procedures on tone rows, and minimalist composers including Steve Reich developed phase pieces where two identical parts start together and gradually drift apart, which is a canon with a continuously changing delay and produces effects impossible with a fixed offset. Pachelbel's canon is the piece most people know by the name, and notably the famous part is the repeating bass line rather than the canon itself, which unfolds in three violin parts above it.
The takeaway
A canon repeats one melody in several voices at a fixed delay, so the composer must write a line that harmonises with itself, and a round is the simplest case looping indefinitely. Variants invert the melody, reverse it, change its speed or combine those, and puzzle canons were notated as one line with a riddle telling the performer how to derive the rest. Bach's Goldberg Variations place a canon at every third variation at successively wider intervals, and Reich's phase pieces are canons with a shifting delay.