A hurdy gurdy is a stringed musical instrument. It consists of several strings arranged such that they can all be vibrated by a rotating wheel covered with rosin. This method of producing sound is similar to string instruments such as the violin; but because the hurdy gurdy produces several notes, a melody accompanied by chords made by "drone strings", its sound is perhaps more comparable to that of bagpipes.

The earliest form of the hurdy gurdy was the organistrum, a large instrument played by two people, one to turn the crank and the other to pull the keys upward. Subsequent types were reduced in size to allow one person to play the instrument and switched the key mechanism to require pushing rather than pulling, a much more elegant mechanism that facilitated more complex playing techniques.

Some types of hurdy gurdy, notably the French vielle and the Hungarian tekerõ, also added a buyying bridge or dog to one or more drone strings. This mechanism consists of a bridge for the drone string that is loosely anchored on one side and free on the end the string rests on. Normally the pressure of the string holds the bridge in place, but when the instrument is cranked with greater velocity, the bridge lifts up and buzzes against the soundboard, producing a characteristic rhythmic buzz that is used to provide percussive effect, especially in dance pieces.

The drone strings just produce sounds at fixed pitches, while the other strings are played by a key mechanism which stops each string, adjusting its length, and hence the pitch.