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.
Bass violin
The viol, or "viola da gamba," was introduced to Italy from Spain around 1490. Before the introduction of the viol, no bowed instrument existed in the region The viol da gamba was played in the "a gamba" position (i.e., between the legs like a cello, as opposed to under the chin like a violin). The "viola da gamba" was also much larger, and therefore could play much lower notes than other fiddles in Italy at that time. The first Italian viols (or ""violoni"" as they were often called) soon began to take on many characteristics of the precursors to the violin, such as separate tail pieces, and arched bridges that let the player sound one string at a time. (Though paintings like Jan Brueghel the Elder's "The Rustic Wedding" and Jambe de Fer in "Epitome Musical" suggest that the bass violin had alternate playing positions, these were short-lived and the more practical and ergonomic "a gamba" position eventually replaced them entirely.) One of the qualities that was almost certainly adopted by the Italian violin makers from the early Spanish viols was the C-bout, which they soon stylized. At some point in the early to mid-sixteenth century, an Italian maker (possibly Amati) set out to create a "violone" that was more closely matched, in appearance, tuning, and number of strings, to the new violin. Judging by artistic representations of the period, this may have been a somewhat gradual development. For example, there are depictions of instruments that appear to be bass violins (such as the one in Gaudenzio Ferrari's "Glory of Angels", 1535), but that clearly show the presence of frets. Once the distinction became clear, and the form of the bass violin had crystallized, theorists and composers began to refer to the new instrument as the ""basso da viola da braccio"", or the first true bass violin.