French horn capable of playing at either of two basic pitches, selected by a valve, usually the 4th (`thumb' or `change') valve. In the full double horn the basic three valves are each equipped with two valve loops and tuning slides of different lengths and two sets of passages in the rotor or piston. The 4th valve determines whether the windway can be directed through the longer or the shorter loops of the basic three valves. In the compensating double horn Compensating valves are used. The usual tonalities for a double horn are 12-ft F + 9-ft B-flat.