Guidonian hand
In Medieval music, the Guidonian hand was a mnemonic device used to assist singers in learning to sight-sing. Some form of the device may have been used by Guido of Arezzo, a medieval music theorist who wrote a number of treatises, including one instructing singers in sightreading. The hand occurs in some manuscripts before Guido's time as a tool to find the semitone; it does not have the depicted form until the 12th century. Sigebertus Gemblacensis in 1105–10 did describe Guido using the joints of the hand to aid in teaching his hexachord. The Guidonian hand is closely linked with Guido's new ideas about how to learn music, including the use of hexachords, and the first known Western use of solfege.
Guido of Arezzo
Guido of Arezzo (also Guido Aretinus, Guido Aretino, Guido da Arezzo, Guido Monaco, 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).