Canon, KAN uhn, is a musical composition in which two or more voices or instruments repeat a melody. One begins the melody and one or more others imitate it. The additional voices or instruments join in at slightly different times and overlap as a result. Songs that are called rounds are the simplest kind of canon. The song "Row, Row, Row Your Boat" is a popular example of a round. Rounds are called perpetual canons because they can go on indefinitely. A canon is a type of counterpoint (see Counterpoint). A canon is one of the most difficult types of counterpoint to compose because the voice or instrument can enter at any interval or on any rhythmic beat.