Guido d'Arezzo (c. 990-c. 1033), music theorist. Educated at the Benedictine Abbey of Pomposa near Ferrara, Italy, he trained the singers and, together with a Brother Michael, created an antiphoner (a collection of antiphons, now lost) using a revolutionary notational system. He moved to Arezzo around 1025, was employed by Bishop Theodaldus to teach the cathedral singers, and wrote his influential treatise, the Micrologus. Around 1028 he was summoned to Rome by Pope John XIX to explain his antiphoner, but due to ill health left in the summer.
The Regulae Rhythmicae, an additional prologue to his antiphoner, explained the colouring of the stave lines c (yellow) and f (red), these being the two degrees of the white-note scale above the semitone intervals (b-c and e-f), which singers had difficulty in locating. The Epistola de Ignoto Cantu, a letter to Brother Michael, describes the still-used teaching device of solmization (Sol-fa), in which the syllables ut, re, mi, fa, sol, and la (derived from a Latin hymn text), denote the degrees of the scale in any key.