In music, a canon is a contrapuntal structure that employs a melody with one or more imitations of the melody played after a given duration. The initial melody is called the leader, while the imitative melody, which is played in a different voice, is called the follower. The follower must imitate the leader, either as an exact replication of its rhythms and intervals or some transformation thereof.  Repeating canons in which all voices are musically identical are called rounds – Sumer Is Icumen In is a good examples.