What unusual musical instrument did Leonardo da Vinci invent?

Leonardo da Vinci invented the viola organista.

The viola organista combines features of a keyboard and bowed string instrument. Pressing keys would select strings, while a rotating wheel or wheels would rub against them continuously, producing sustained tones. Leonardo sketched several versions of the design in the late fifteenth and early sixteenth centuries.

Unlike a conventional organ, the instrument was not primarily based on pipes. Unlike a violin, it did not require a performer to move a bow by hand across each string. The concept sought to give a keyboard player the expressive, sustained sound associated with bowed instruments.

Leonardo’s instrument was never known to have been completed during his lifetime. Modern builders have produced playable versions, including instruments based on his drawings. Its name is a modern descriptive label rather than a term that Leonardo consistently used for one finalized model.