The organistrum, an early, larger form of the hurdy-gurdy (possibly inspired by the fiddle), shaped like a guitar but with a long, wide neck for keys, requiring two players (one pulled keys in an upward motion to change the pitch of the melody string while the other cranked a small wheel to produce a continuous sound on two drone strings). Thought to be the first use of the principle of a keyboard. A 13th century treatise on the construction of an organistrum, describes an instrument having eight tangents set at intervals based on Pythagorean principles.