A musical form in which a tune is imitated by individual parts at regular intervals; known as a round when each part is continuously repeated. In simple examples, such as “London Bridge is Falling Down,” the successive voices enter at a same pitch and at the same speed. In more elaborate examples, such as the canons in J.S. Bach’s keyboard work known as the Goldberg Variation, the voices may enter at different pitches and present the tune at different speeds or even backwards or upside down.