What instrument did Leonardo da Vinci design that used keys to bow strings continuously?

Leonardo da Vinci designed the viola organista, a keyboard instrument that used rotating wheels to bow strings continuously.

Leonardo sketched the concept in the late 15th century. Pressing a key would select a string, while a wheel covered with horsehair or another friction material would act like a moving bow. The idea combined the keyboard’s ability to play several notes with the sustained sound of bowed string instruments.

The viola organista was never built by Leonardo, so it is better described as a design than as a completed Renaissance invention. Modern makers have constructed working versions from his drawings. It is sometimes confused with the hurdy-gurdy, which also uses a wheel against strings, but the hurdy-gurdy is played with a crank and stopped keys rather than Leonardo’s proposed keyboard mechanism.