Guido of Arezzo: Micrologus de Disciplina Artis Musicae

The Micrologus is Guido of Arezzo’s principal treatise on music, written for singers of chant. It opens with a letter to Theodald, bishop of Arezzo, in which Guido explains that, though he wished to live a solitary life, the bishop had associated him with the work of teaching in his church, and that he had laboured on the practice of music to the benefit of the clergy. A prologue and list of chapters follow. The treatise covers the notes of the monochord, intervals, the construction of melody and the modes. In the sample Guido explains how the four ancient modes, each divided into authentic and plagal, became the eight church modes numbered first to eighth.

The Micrologus was one of the most widely copied music treatises of the Middle Ages, second only to Boethius as a school authority. It is important for its practical approach to teaching chant, its discussion of modal character, and its early account of organum, the practice of adding a second voice to the chant. It remains a foundational text for the history of medieval music theory.