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In music rondellus is the formalized interchange of parts or voices according to a scheme, often used in English conducti and frequently in English motets of the late thirteenth and early fourteenth centuries, but never used for an entire piece (Caldwell 1992, p. 47). For example:

A B C  D E F
C A B  F D E
B C A  E F D

where the italicized letters represent music with text and the other letters are melismatic (Caldwell 1992, p. 47).

Sources

Caldwell, John (1992), The Oxford History of English Music: Volume 1: From the Beginnings to c.1715, Oxford and New York: Ocford University Press, ISBN 0-19-816129-8

Falck, Robert (1972). "Rondellus, Canon, and Related Types before 1300". Journal of the American Musicological Association 25, no. 1 (Spring): 38–57.
Harrison, Frank Lloyd. (1959–60). "Rota and Rondellus in English Medieval Music". Proceedings of the Royal Musical Association 86:98–107.
Sanders, Ernest. H. (2001). "Rondellus". The New Grove Dictionary of Music and Musicians, second edition, edited by Stanley Sadie and John Tyrrell. London: Macmillan Publishers.