Guido d'Arezzo (ca. 995-ca. 1050) was an Italian music theorist and pedagogue who developed the hexachord system and the musical staff.
Guido d'Arezzo was probably born in Italy, although it has been conjectured that he may have come to Italy from France at an early age. He studied at the Benedictine Abbey of Pomposa and then taught singing there. He left the abbey about 1025 because his ideas did not meet with understanding. The bishop of Arezzo invited him to teach music at his cathedral school and became a great admirer of Guido's new pedagogic devices. These were incorporated in Guido's famous textbook, Micrologus, written about 1030.
At Pomposa, Guido had developed a new way of writing Gregorian chant, adopting a four-line staff and clefs. He explained his new methods in the foreword to his antiphonal, a volume of chants that he rewrote in his new way during the 1020s and presented to Pope John XIX, who was greatly impressed. This system of notation is the direct ancestor of all subsequent musical notation.
Educator that he was, Guido developed this idea further in a complete system of ear training and sight singing, which he explained in a letter written from Arezzo to the monk Michael at Pomposa. This system, known as solmization, became the basis of modern solfeggio. For a well-known 8th-century hymn for the feast of St. John the Baptist, Guido created a melody, the first notes of whose first six lines form a scale of two whole tones, a half tone, and two whole tones. This symmetric series of six notes, called a hexachord and sung to the Latin syllables that begin the six lines, ut-re-mi-fa-sol-la, became his central tool for ear training and sight singing. By shifting this hexachord to various pitch levels, the singer could always determine where the crucial half-tone interval must be sung.
Guido or one of his disciples also invented a memory aid for learning the names of all the notes in his musical system, which extends over a range of 20 white keys on the piano:the so-called Guidonian hand. Here the note names of the system were written on the various portions of the left hand and fingers, so that they could be read off by the pupil.
Guido's fame was great, and his ideas had a lasting influence on musical notation, music teaching, and musicianship. In these fields he was one of the most outstanding men in all of Western music.
The best account of Guido d'Arezzo is in Donald J. Grout, A History of Western Music (1960). See also the fuller treatment of him in Gustave Reese, Music in the Middle Ages (1940).