Guido of Arezzo (ca. 991–1033) was a Benedictine monk and music theorist, best known for developing the modern system of musical notation and solmization (the basis of solfège). Born in Arezzo, he entered the monastery of Pomposa near Ferrara, where he wrote his influential treatise Micrologus (ca. 1026). His letter to Archbishop Heribert of Milan against simony, written in the early eleventh century, is one of the earliest systematic reform-era arguments against the purchase of holy orders, drawing on Gregory the Great and Augustine to argue that simoniacal ordination communicates no valid grace and that complicity in simony amounts to heresy.
