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.
Viola organista
Georg Gandi, an organist in Ilmenau, constructed a similar instrument with some improvements in 1709 and called it the "piano viol". In 1741, two inventors—Le Voir in Paris and Hohlfeld in Berlin—independently constructed versions of a "bowed piano", similar to Heyden's design. In 1754 a horsehair covering was added to the wheels of the bowed piano, which was termed the "viola da gamba" piano. Other instrument makers continued to improve the design through the remainder of the 18th and into the early 19th centuries, though it never became widespread or common.