Léonin
French 12th-century composer associated with polyphony in Notre Dame
Léonin (lived in the late 12th century) is the first composer we know about who wrote organum, a kind of church music. We know nothing about his life. He was probably French. He seems to have worked at Notre Dame Cathedral because he wrote a collection of music called Magnus Liber (meaning: Big Book), or, at least, he wrote some of it. This book has music for two voices. It was designed for services at Notre Dame. A younger composer called Pérotin did some more work on the collection, changing some of it.
References
change- The New Grove Dictionary of Music and Musicians, ed. Stanley Sadie; 1980. (ISBN 1-56159-174-2)