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Canons and fugues are highly structured contrapuntal forms in which melodic material is restated and developed through systematic imitation. In a canon, one voice presents a melody that subsequent voices replicate at a time interval, creating both unity and independence through constrained repetition. Fugues extend this principle with subject statements in multiple voices, development sections, and sophisticated contrapuntal combinations. Understanding these forms reveals how compositional unity emerges from the disciplined treatment of a single melodic idea.
Canon and fugue build directly on two-voice counterpoint. In counterpoint, you learned to write two independent melodic lines that work together harmonically while each makes sense on its own. Canon and fugue take that independence and add a new organizing principle: imitation, where one voice restates what another voice just sang or played. The compositional discipline is that the melody must be its own counterpoint — whatever the leading voice plays must harmonize with what the following voice will be playing at the same time.
A canon is the simplest form of imitative counterpoint. One voice (the dux, or leader) presents a melody. After a fixed time interval, a second voice (the comes, or follower) begins the exact same melody, while the first voice continues. You already know the most familiar example: "Row, Row, Row Your Boat" sung as a round. Each entrance works because the melody was constructed so that its opening harmonizes with its middle and its middle harmonizes with its ending. Canon makes this constraint explicit and total — the composer must write a single line that counterpoints itself at a specified interval of time and pitch.
A fugue extends this logic into a full formal structure. The subject — a short, distinctive melodic idea — is announced alone in one voice, then imitated by a second voice (the answer) while the first continues with a countersubject. Once all voices have entered in the exposition, the fugue opens into episodes (passages where the subject is absent, often using sequence and fragmentation) and further entries that restate the subject in new keys. The distinguishing feature of fugue is not just imitation but the systematic harmonic journey of a single subject through development, combination, and return.
The deepest technical requirement is invertible counterpoint: the subject and countersubject must be designed so that either can be placed on top without violating voice-leading rules. When the two parts swap registers, the intervals between them change (a sixth becomes a third, a fifth becomes a fourth), and what worked as a consonance can become a dissonance. Fugue composers design subject-countersubject pairs with this inversion in mind from the start — a discipline that forces long-range thinking and gives fugues their characteristic contrapuntal density.
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