A thousand years after being written, the theories of Guido d'Arezzo (Guido Aretinus) have thoroughly affected modern systems of notation and music education. Born sometime in the last decade of the first millennium, he received his education and training as a monk at the Benedictine abbey in Pomposa, Italy, on the northeast coast, near Ferrara and Ravenna.
While in Pomposa, influenced by the Dialogus attributed to Odo, d'Arezzo developed an innovation in staff notation that had previously consisted of only two lines. D'Arezzo added a red line for "f" (fa) and a yellow line for "c" (do); the result was to make notation clearer and more exact, as well as to create a staff system divided by lines at the interval of a third. D'Arezzo himself noted how much more quickly singers were able to learn chant with his method; however, as he began to receive recognition from the rest of Italy, he inspired the jealousy of his fellow monks, who likely forced d'Arezzo out.
From there, he went to the cathedral school at Arezzo (Aretium), where he began to work training chant singers for Bishop Theobald (Theodaldus), who greatly admired his new method. For Theobald he wrote the Micrologus de Disciplina Artis Musicae (A Short Treatise on the Discipline of Musical Art) around 1026. Addressed to singers, the Micrologus attempted to teach them his new notation and build their skill at learning unfamiliar chant. His gamut (scale) of 21 notes went farther than the scale of the earlier Dialogus and he also outlined his theory of the eight church modes. In a striking passage on melody, d'Arezzo compared the composition of a melodic line, note, phrase, and line, to the phrasing of poetic verse, syllable, word, and line. His chapter on organum, or chant for two or more voices, is an important early source on the practice. Here, d'Arezzo emphasized the "occursus," the process by which voices diverge to the interval of a fourth and come together to a unison; a hierarchy of intervals is also posited. Interestingly, he posited a more flexible ("molle") style of organum, justifying it not on the grounds of avoiding the tritone ("diabolus in musica") as before, but for its own sake as a freer style.
Pope John XIX, intrigued by reports of the treatise, invited d'Arezzo to Rome to explicate it in 1028. In the same year, d'Arezzo wrote out the Micrologus in verse and included examples of his new notation, thus creating a more congenial "textbook" form for educational purposes; unfortunately, the "Regulae rhythmicae" has not survived. Later in life, he became a prior of the Camaldolite fraternity, an order of hermits and cenobites, and died after 1033, perhaps as late as 1056.
In his Epistola de ignoto cantu (Letter on the ignorance of the singer) to his fellow monk Michael, with whom he had apparently written a now-lost antiphonary in their new notation, d'Arezzo remarked that his success as theorist and pedagogue had made him as much martyr as beloved and famously branded singers "the most stupid men of our times." More importantly, however, d'Arezzo spoke of the success he achieved by teaching singers the hexachord scale using the hymn to John the Baptist's Ut queant laxis. Though he did not, as has been sometimes attributed to him, invent the process of solmization, it was this step that paved the way for the later innovation in sightsinging. D'Arezzo's innovations were not entirely new and he built on the work of earlier theorists. Nevertheless, his adaptations were sufficient to ensure that, as Smits von Waesberghe wrote, "We are still in the age of Guido."