What musical instrument did Leonardo da Vinci design using wheels to move bowing strings?

Leonardo da Vinci designed the viola organista, a keyboard instrument whose wheels bow strings to produce sound.

Leonardo developed several versions of the concept during the late fifteenth and early sixteenth centuries. Pressing a key would move a string against a rotating wheel or wheels, allowing the instrument to combine keyboard control with the sustained sound of bowed strings.

The instrument was never completed by Leonardo, and no surviving original example is known. Modern builders have made reconstructions based on his drawings, showing that the mechanism can work.

Despite its name, the viola organista is not simply a viola played by a keyboard. It is closer to a mechanical string instrument, with the keyboard controlling which strings contact the rotating bowing surface.