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.
Dionisio Aguado y García
Aguado's major work "Escuela de Guitarra" was a guitar tutorial published in 1825. As of 2011, it is still in print, with Tecla Editions releasing a reprint in 2005. In the "Escuela" Aguado describes his use of fingernails on the right hand as well as his invention of a "tripodison": a device that held the guitar and thus minimized the damping effect of the player's body on the guitar's back and sides. Aguado's other works include "Trois Rondos Brillants" (Opus 2), "Le Menuet Affandangado" (Opus 15), "Le Fandango Varie" (Opus 16), as well as numerous waltzes, minuets, and other light pieces. The more extended works require a virtuoso technique and left-hand stretches that are almost impossible on the longer string lengths of modern guitars. (See Frederick Noad, "The Classical Guitar")