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Canons are compositions where melodic material enters successively in different voices, creating strict imitative texture. Fugues extend this principle with multiple subjects, episodes, and complex contrapuntal development. Both forms demand careful voice leading to ensure harmonic coherence despite the strict imitative rules.
A canon is counterpoint taken to its logical extreme: the imitation is not just occasional but continuous throughout the entire piece. From your work on counterpoint imitation, you know how to write a phrase in one voice and echo it in another. A canon formalizes this into a governing rule — the dux (leader) states the melody, and the comes (follower) enters at a fixed time interval and pitch interval, reproducing every note exactly. The whole piece is one line heard against itself displaced in time, which means every measure of the leader is simultaneously in counterpoint with an earlier measure of the same melody in the follower.
The practical challenge is that you cannot write the dux without already thinking about how it will sound against itself. When you write bar 3, the comes is playing bar 1. Every vertical sonority must be consonant by the rules of counterpoint you already know — but now the two voices aren't independently composed, they are the same melody offset. This constraint is demanding and creative simultaneously: it forces economy and ingenuity in the melodic line, since every interval and rhythm must work both horizontally (as melody) and vertically (as counterpoint against itself).
Fugue extends the imitative principle but relaxes the strictness. A fugue opens with a subject — a melodic idea stated alone. The subject is then answered in another voice, typically at the pitch level of the dominant (the answer). After all voices have entered in this way, the opening exposition is complete. What follows alternates between full entries (where the complete subject returns, often in new keys) and episodes (transitional passages that develop fragments of the subject or a countersubject — a second melody that habitually accompanies the subject). Episodes are the fugue's connective tissue, and they typically use sequence (a pattern repeated at successive pitch levels) to drive harmonic motion between key areas.
The connecting thread between canon and fugue is that both turn a single melodic idea into a full multi-voice texture through imitative logic. Canon achieves this with mechanical precision; fugue achieves it with flexible developmental judgment. Both forms reward composers who invest in strong initial material, because the imitative machinery amplifies and exposes every melodic interval and rhythmic figure across the full texture — weakness in the subject becomes weakness everywhere.
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