A cancer canon or retrograde canon is a musical palindrome , that is, a canon that sounds the same backwards as it does forwards. One voice carries the melody forward, the other voice carries it backwards, in cancer . In the mirror canon or inverse canon, on the other hand, the mirror is not mirrored vertically, but horizontally so that the voices play the intervals in opposite directions ( inversion ). Another form that combines both is the mirror crab or table canon , in which the singers or players sit opposite one another and have to begin in the opposite corners of the same sheet of music. This retrograde-inverse canon is then not axially, but point-symmetric to the middle of the sentence.
The motif of the crab canon was later also playfully represented in graphic form by MC Escher .