A musical line which is the reverse of a previously or simultaneously stated line is said to be its
retrograde or
cancrizans ("walking backward", medieval Latin, from
cancer, crab). An exact retrograde includes both the pitches and rhythms in reverse. An even more exact retrograde reverses the physical contour of the notes themselves, though this is possible only in
electronic music. Some composers choose to subject just the pitches of a musical line to retrograde, or just the rhythms. In
twelve-tone music, reversal of the
pitch classes alone—regardless of the
melodic contour created by their
registral placement—is regarded as a retrograde.