Guido d'Arezzo gwē´dō därĕt´tsō [key] or Guido Aretinus ârətī´nəs [key], 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 , an early form of polyphony. His work in musical notation 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.
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