Leonardo da Vinci invented the viola organista, a keyboard instrument whose strings were sounded by rotating, continuously bowed wheels.
Leonardo drew several versions of the idea, especially in the Codex Atlanticus and the Codex Madrid II. A keyboard would select the strings, while a crank or pedal would turn a wheel covered with hair or another friction material. The wheel would act like a bow, allowing sustained notes without a player moving a separate bow by hand.
The viola organista was not a normal organ, despite its name. It used strings rather than pipes, and its sound-producing mechanism combined features associated with bowed instruments and keyboard instruments. Leonardo's surviving designs vary, so there was not one single final model.
No evidence shows that Leonardo completed a working instrument during his lifetime. Several modern builders have produced reconstructions, demonstrating that the principle can work. The invention is sometimes confused with the piano, but a piano strikes strings with hammers, whereas the viola organista produces sound through friction from rotating bowed surfaces.