Leonardo da Vinci designed the viola organista, a keyboard instrument in which a rotating wheel bows strings.
Leonardo explored the idea in several drawings from the late 15th and early 16th centuries. Pressing keys would select strings, while a wheel or wheels coated with material such as horsehair would rub against them to produce sound. The concept combined the keyboard's ability to play several notes with the sustained tone of a bowed string instrument.
Unlike a conventional violin, the player would not move a bow directly across the strings. The mechanism was intended to provide continuous friction, allowing the instrument to sustain notes while the keyboard controlled pitch selection.
No fully documented Leonardo-built viola organista survives. Modern makers have constructed versions based on his drawings, and some have demonstrated that the principle can produce music. The invention is often confused with an organ because of its keyboard, but its sound-producing mechanism is based on bowed strings rather than pipes.