A crab canon (also known by the Latin form of the name, canon cancrizans; as well as retrograde canon, canon per recte et retro or canon per rectus et inversus) is an arrangement of two musical lines that are complementary and backward. If the two lines were placed next to each other (as opposed to stacked), the lines would form something conceptually similar to a palindrome. The name 'crab' refers to the fact that crabs are known to walk backward (although they can also walk forward and sideways). It originally referred to a kind of canon in which one line is played backward (e.g. FABACEAE played simultaneously with EAECABAF). An example is found in J. S. Bach's The Musical Offering, which also contains a table canon ("Quaerendo invenietis"), which combines retrogression with inversion by having one player turn the music upside down.
- Riemann, Hugo (1904). Text-book of simple and double counterpoint including imitation or canon, p.185. Breitkopf & Härtel.
- Kennedy, Michael (ed.). 1994. "Canon". The Oxford Dictionary of Music, associate editor, Joyce Bourne. Oxford and New York: Oxford University Press. ISBN 0-19-869162-9.