A round or perpetual canon is a musical composition, a limited type of canon, in which a minimum of three voices sing exactly the same melody at the unison (and may continue repeating it indefinitely), but with each voice beginning at different times so that different parts of the melody coincide in the different voices, but nevertheless fit harmoniously together . It is one of the easiest forms of part singing, as only one line of melody need be learned by all parts, and is part of a popular musical tradition. They were particularly favoured in glee clubs, which combined amateur singing with regular drinking (, especially at 21: "Catch-singing is unthinkable without a supply of liquor to hand..."). The earliest known rounds date from the 12th century.