A crab canon is an arrangement of two things that are complementary and backward, similar to a palindrome. Originally it is a musical term for a kind of canon in which one line is reversed in time from the other (e.g. FABACEAE <=> EAECABAF). A famous example is found in J. S. Bach's A Musical Offering (BWV 1079), which also contains a canon ("Quaerendo invenietis") combining retrogresion with inversion, i.e., the music is turned upside down by one player. The use of the term in non-musical contexts was popularized by Douglas Hofstadter in Gödel, Escher, Bach.