Léonin (active ca. 1165-1185), or Leoninus, of the Cathedral of Notre Dame in Paris, is the earliest known composer of polyphonic art music and the creator of controlled rhythm and meter, as well as of the earliest notation to convey rhythm.

About the life of Léonin absolutely nothing is known. His name is mentioned in a treatise, actually class notes taken at lectures an anonymous English student attended at the University of Paris about a century later, in the 1270s. In this treatise Léonin is connected with Paris and is praised as the best composer of organa (two-voiced settings of soloistic portions of chants of the Mass and the daily prayer hours).

Léonin evidently composed his organa for the Cathedral of Notre Dame, whose present magnificent stone structure rose in the main between 1163 and 1208. It has been suggested that he...