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.
Hand of God (art)
In Eastern Orthodox icons the hand remained in use far longer than in the Western church, and is still found in modern icons, normally emerging from circular bands. Apart from the narrative scenes mentioned above it is especially often found in icons of military saints, and in some Russian icons is identified by the usual inscription as belonging to Jesus Christ. In other versions of the same composition a small figure of Christ of about the same size as the hand takes its place, which is also seen in many Western works from about 1000 onwards.
The earliest surviving icon of the Virgin Mary, of about 600 from Saint Catherine's Monastery, has an often overlooked hand, suggesting to Robin Cormack that the emphasis of the subject is on the Incarnation rather than a simple Virgin and Child. Another of the very few major Eastern works showing the Virgin from before the Byzantine iconoclasm, an apse mosaic (lost in 1922) from Nicaea, also shows the hand above a standing Virgin. Few similar uses of the hand are seen in later Virgins, though the iconographically adventurous Byzantine Chludov Psalter (9th century) has a small miniature showing the hand and dove above a Virgin & Child. The hand occasionally appears in Western "Annunciations", even as late as Simone Martini in the 14th century, by which time the dove, sometimes accompanied by a small image of God the Father, has become more common.
The hand appears at the top of a number of Late Antique apse mosaics in Rome and Ravenna, above a variety of compositions that feature either Christ or the cross, some covered by the regular contexts mentioned above, but others not. The motif is then repeated in much later mosaics from the 12th century.