Strict compositional technique; a piece of music, in which different voices or instruments successively take up the same theme at fixed time intervals, giving rise to a polyphonic composition. A canon is usually notated in one part, and the entrance of the other parts is marked at given points. In the case of a riddle canon these marks are missing. The musical interval in which the other parts are to enter and their starting point have to be guessed (to be solved like a riddle). Beethoven wrote 40 canons, mostly spontaneously as greetings, entries in albums, New Years greetings or musical jokes. Ten of them are in the form of riddle canons.