Career
He is known for his work on the first treatise to explore the practice of musical notation of rhythm, De mensurabili musica. Prior to this time, music notation applied to pitch only. Until the mid-1980s it was believed that Johannes de Garlandia lived in the first half of the 13th century and wrote two treatises, De Mensurabili Musica and De plana musica, and thus was intimately connected with the composers of the Notre Dame school, at least one of whom — Pérotin — may still have been alive in the earlier part of his career.
Unfortunately the linking of his name with those two works only began after 1270, and it now seems likely that Garlandia was one Jehan de Garlandia, a keeper of a bookshop in Paris, records of whom appear on various official Parisian documents between 1296 and 1319.
Most likely he was an editor of the two previous anonymous treatises, and while he did much to clarify them and transmit to them to posterity, he did not write them.