Leonardo da Vinci's viola organista used rotating wheels to make strings vibrate, combining features of a keyboard and bowed instrument.
The instrument appears in Leonardo's notebooks and was designed to be played from a keyboard. Pressing a key would move a string against a continuously rotating wheel, creating friction and sound in a manner related to a bow moving across strings. Several possible arrangements appear in his drawings, so the surviving designs are not a single completely standardized instrument.
The name viola organista reflects its hybrid character. It had the keyboard control and sustained action associated with an organ, while its sound production depended on vibrating strings. Leonardo's concept therefore differs from a pipe organ and from his separate barrel-organ designs.
Leonardo never appears to have completed a surviving original viola organista. Modern instrument makers have constructed versions based on his sketches. These reconstructions show that the idea is mechanically feasible, although their sound depends on modern interpretations of the incomplete plans.