Guido d'Arezzo (ca. 995-ca. 1050) was an Italian music theorist and pedagogue who developed the hexachord system and the musical staff.

Guido d'Arezzo was probably born in Italy, although it has been conjectured that he may have come to Italy from France at an early age. He studied at the Benedictine Abbey of Pomposa and then taught singing there. He left the abbey about 1025 because his ideas did not meet with understanding. The bishop of Arezzo invited him to teach music at his cathedral school and became a great admirer of Guido's new pedagogic devices. These were incorporated in Guido's famous textbook, Micrologus, written about 1030.

At Pomposa, Guido had developed a new way of writing Gregorian chant, adopting a four-line staff and clefs. He explained his new methods in the foreword to his antiphonal, a volume of chants that he rewrote in his new way during the 1020s and...