In a series of pieces written for King Frederick II of Prussia in 1747 called The Musical Offering, Johann Sebastian Bach included a canon that is popularly referred to as the Crab Canon. The piece is a puzzle to be worked out by the reader/player.
You may notice that the Crab Canon is performed by two instruments, but only one line is notated. What’s the deal?
Bach published the canons in the Musical Offering as puzzles, giving the reader the minimum amount of information with which they can figure out the piece as long as they understand its structure. To “solve” a puzzle canon is to give it a structure that makes it fit together in pleasing harmony.
The solution to the Crab Canon is that it can be played forwards or backwards or forwards and backwards together in accompaniment. It’s a musical palindrome of sorts. (via open culture)