Crab canon
A canon whose following voice performs the melody backwards.
What it means
A crab canon is a canon in which the second voice performs the same melody as the first, but backwards, from the last note to the first. Together the two voices form a coherent contrapuntal texture.
Musical use
The crab, or cancrizans, technique belongs to the strict contrapuntal art forms and was especially popular in Renaissance and Baroque music, where it was prized as an intellectual game and a display of compositional mastery; a well-known example appears in Bach's Musical Offering. Since both voices can come from the same line of notation simply read in opposite directions, a crab canon can often be played by two musicians reading the same page from opposite ends. A crab canon differs from a simple canon, whose following voice repeats the melody unchanged and forwards, and from an inversion canon, which mirrors the melody's pitch contour rather than reversing it in time.
Performance practice
When learning a crab canon, practise both directions of the melody separately before putting them together — reading it backwards demands its own, unfamiliar habit of sight-reading.