Guido d'Arezzo (gwē`dō därĕt`tsō) or Guido Aretinus (ârətī`nəs), c.990–1050, Italian Benedictine monk, known for his contributions to musical notation and theory. His theoretical work Micrologus (c.1025) is one of the principal sources of our knowledge of organum organum (ôr`gənəm), in music, compositional technique, developed in Europe during the 10th cent.
..... Click the link for more information. , an early form of polyphony. His work in musical notation musical notation, symbols used to make a written record of musical sounds.

Two different systems of letters were used to write down the instrumental and the vocal music of ancient Greece. In his five textbooks on music theory Boethius (c.A.D. 470–A.D.
..... Click the link for more information. included the addition of two lines (one red, one yellow) to the two already serving as a staff and the use of both the lines and the spaces. Also important was his system of solmization (sometimes called, after him, Aretinian syllables), whereby the syllables ut, re, mi, fa, sol, la are used as names for the six tones, C to A, known as the hexachord. As the octave replaced the hexachord, an additional syllable, si or ti, was added, and eventually ut was replaced by the more singable do. Other revisions of Guido's system that have been suggested from time to time have not survived.

Guido d'Arezzo

(born c. 990, Arezzo? —died 1050, Avellana?) Italian music theorist. A Benedictine monk charged with training the choristers in the cathedral of Arezzo, he is credited with two important advances: the invention of the staff for notating exact pitches, and the use of different syllables to sing each pitch, or solmization. In his famous Micrologus (1026–33), he describes the use of at least a two-line staff and the use of syllables as a mnemonic device for singing musical pitches. The famous “Guidonian hand,” an aid to modulation from one hexachord to another by using the joints of the hand to represent different pitches, is not mentioned in his extant writings.