The horn (informally known also as the French horn) is a brass instrument, descended from the natural horn that consists of about 12 feet of tubing (for a single horn in the key of F), wrapped into a compact, coiled form with a flared bell.

Most horns have finger-operated valves (a horn without valves is known as a natural horn, and some horns, such as the vienna horn, use piston valves).

A single horn, which will usually be tuned to either F or B flat, has three valves; the more common double horn has two sets of tubing (generally F and B flat) and a fourth valve, operated by the thumb, which routes the air flow to one or other set. Triple horns, with five valves, are also made.