Guido of Arezzo (also Guido Aretinus, Guido Aretino, Guido da Arezzo, Guido Monaco, or Guido d'Arezzo, or Guy of Arezzo also Guy d'Arezzo) (991/992 – after 1033) was an Italian music theorist of the Medieval era. He is regarded as the inventor of modern musical notation (staff notation) that replaced neumatic notation; his text, the Micrologus, was the second-most-widely distributed treatise on music in the Middle Ages (after the writings of Boethius).
Guido was a monk of the Benedictine order from the Italian city-state of Arezzo. Recent research has dated his Micrologus to 1025 or 1026; since Guido stated in a letter that he was thirty-four when he wrote it, his birthdate is presumed to be around 991 or 992. His early career was spent at the monastery of Pomposa, on the Adriatic coast near Ferrara. While there, he noted the difficulty that singers had in remembering Gregorian chants.
He came up with a method for teaching the singers to learn chants in a short time, and quickly became famous throughout north Italy. However, he attracted the hostility of the other monks at the abbey, prompting him to move to Arezzo, a town which had no abbey, but which did have a large group of cathedral singers, whose training Bishop Tedald invited him to conduct.
While at Arezzo, he developed new techniques for teaching, such as staff notation and the use of the "ut–re–mi–fa–so–la" (do–re–mi–fa–so–la) mnemonic (solmization). The ut–re–mi-fa-so-la syllables are taken from the initial syllables of each of the first six half-lines of the first stanza of the hymn Ut queant laxis, whose text is attributed to the Italian monk and scholar Paulus Diaconus (though the musical line either shares a common ancestor with the earlier setting of Horace's "" (Odes 4.11) recorded in the Montpellier manuscript H425, or may even have been taken from it.)Giovanni Battista Doni is known for having changed the name of note "Ut" (C), renaming it "Do" (in the "Do Re Mi ..." sequence known as solfège). A seventh note, "Si" (from the initials for "Sancte Iohannes," Latin for St. John the Baptist) was added shortly after to complete the diatonic scale. In Anglophone countries, "Si" was changed to "Ti" by Sarah Glover in the nineteenth century so that every syllable might begin with a different letter (this also freed up Si for later use as Sol-sharp). "Ti" is used in tonic sol-fa and in the song "Do-Re-Mi".