A mechanical Fiddle in which the bow is replaced by a rosined wheel and the bowing skill is replaced by keys and the skill of winding the wheel correctly. They are used widely in French music where they are called Vielle a Roue - Wheel Fiddle.
Hurdy Gurdys now usually have 24 keys (2 octaves) which makes them fully Chromatic. Each key presses a wooden (usually) wedge called a tangent against a string to stop the note. The use of Drone strings gives a continuous sound making the instrument a substitute for Bagpipes. The Instrument usually has a buzzing Bridge (dog or chien) under the highest pitch drone string (Trompette) which vibrates as the wheel accelerates and causes the characteristic buzz. The string passes over the dog bridge which is fitted loosely in the bottom of the Mouche bridge (the next lowest drone). The Melody (Chanter) strings pass through the keybox and the drone strings pass outside. Each string has a thin cotton covering where it contacts the wheel to prevent it wearing through.
There are two chanter strings tuned to g', two Bass Drones (Petit and Gros Bourdon) tuned to c and G respectively, a high drone (Tenor or Trompette) tuned to c' or g' and an intermediate (Mouche) tuned to g. This GC tuning is known as Auvergnat while a DG tuning is known as Bourbonnais. There are three bridges - one for each pair of drones and one for the two chanters.