Leonardo da Vinci designed the viola organista, a keyboard instrument using rotating wheels to bow strings continuously.
The instrument combines features of a keyboard and a bowed string instrument. Pressing keys would select strings, while rotating wheels or belts would provide the friction normally supplied by a bow. Leonardo made several sketches of related mechanisms, showing his interest in producing sustained musical tones through machinery.
The viola organista was not a conventional organ, despite its name. It produced sound from strings rather than pipes, and its keyboard controlled the selection and stopping of notes. The design was technically ambitious and was not completed as a working instrument by Leonardo.
Modern builders have constructed playable versions based on his drawings. These reconstructions demonstrate the principle, but they should not be confused with a surviving original instrument made by Leonardo.