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Descended from the natural horn, this instrument is informally known as the French horn. They have valves, operated with the left hand, to route the air into extra tubing to change the pitch. Most horns have lever-operated rotary valves, but some horns, like the Vienna horn, use piston valves, which are similar to trumpet valves. A horn without valves is known as a natural horn, changing pitch along the natural harmonics of the instrument, but with a wide range of notes due to the long tubing.

Three valves control the flow of air in the "single horn," which is tuned to F or less commonly, B♭. The more common "double horn" has a fourth vavle, usualy operated by the thumb, which routes air to one set of tubing tuned to F or the second set of tubing tuned to B♭. "Triple horns" with five valves are also made, tuned in F, B♭, and a descant E♭ or F.

Source: wikipedia.org
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