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.
Gittern
The gittern was a relatively small gut strung round-backed instrument that first appears in literature and pictorial representation during the 13th century in Western Europe (Iberian Peninsula, Italy, France, England). It is usually depicted played with a quill plectrum, as we can see clearly beginning in manuscript illuminations from the thirteenth century.  
It was also called the guiterna in Spain, guiterne or guiterre in France, the chitarra in Italy and quintern in Germany. A popular instrument with court musicians, minstrels, and amateurs, the gittern is considered ancestral to the modern guitar and possibly to other instruments like the mandore and gallichon.