Leonardo da Vinci designed the viola organista as a keyboard instrument that produced sound with bowed strings.
The player would press keys, while a mechanism moved a rotating wheel or series of wheels against strings, imitating the sustained tone of bowed instruments. Leonardo sketched several versions, especially during the late 15th century.
The design combined keyboard control with the sound-producing principle of a violin or viola. Unlike a conventional keyboard, it was intended to let a performer vary or sustain tones rather than simply strike strings.
Leonardo never completed a known working instrument. Modern builders have reconstructed versions from his drawings, but these reconstructions should not be confused with an original surviving instrument.