Guido of Arezzo was an Italian music theorist and pedagogue of High medieval music. A Benedictine monk, he is regarded as the inventor—or by some, developer—of the modern staff notation that had a massive influence on the development of Western musical notation and practice. Perhaps the most significant European writer on music between Boethius and Johannes Tinctoris, after the former's De institutione musica, Guido's Micrologus was the most widely distributed medieval treatise on music.

In music theory, the term mode or modus is used in a number of distinct senses, depending on context.