Double principle

System of constructing brasswind instruments whereby two valves (on three-valve instruments) or three valves (on four-valve instruments) are equipped with two sets of valve loops and tuning slides in order to give better intonation when valves are used in combination. This is the principle of the Double horn, and Besson's Enharmonic Patent valves; it was also used for some small brass instruments by Boosey & Co marked COMPENSATING PISTONS but not strictly Compensating valve at all