The French horn (since the 1930s known simply as the horn in professional music circles) is a brass instrument made of tubing wrapped into a coil with a flared bell. The double horn in F/B♭ (technically a variety of German horn) is the horn most often used by players in professional orchestras and bands, although the descant and triple horn have become increasingly popular. A musician who plays a horn is known as a horn player or hornist.

Pitch is controlled through the combination of the following factors: speed of air through the instrument (controlled by the player's lungs and thoracic diaphragm); diameter and tension of lip aperture (by the player's lip muscles—the embouchure) in the mouthpiece; plus, in a modern horn, the operation of valves by the left hand, which route the air into extra sections of tubing. Most horns have lever-operated rotary valves, but some, especially older horns, use piston valves (similar to a trumpet's) and the Vienna horn uses double-piston valves, or pumpenvalves. The backward-facing orientation of the bell relates to the perceived desirability to create a subdued sound in concert situations, in contrast to the more piercing quality of the trumpet. A horn without valves is known as a natural horn, changing pitch along the natural harmonics of the instrument (similar to a bugle). Pitch may also be controlled by the position of the hand in the bell, in effect reducing the bell's diameter. The pitch of any note can easily be raised or lowered by adjusting the hand position in the bell. The key of a natural horn can be changed by adding different crooks of different lengths.

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, trigger valve, usually operated by the thumb, which routes the air to one set of tubing tuned to F or another tuned to B♭ which expands the horn range to over four octaves and blends with flutes or clarinets in a woodwind ensemble. Triple horns with five valves are also made, usually tuned in F, B♭, and a descant E♭ or F. There are also double horns with five valves tuned in B♭, descant E♭ or F, and a stopping valve, which greatly simplifies the complicated and difficult hand-stopping technique, though these are rarer. Also common are descant doubles, which typically provide B♭ and alto F branches.

A crucial element in playing the horn deals with the mouthpiece. Most of the time, the mouthpiece is placed in the exact center of the lips, but, because of differences in the formation of the lips and teeth of different players, some tend to play with the mouthpiece slightly off center. Although the exact side-to-side placement of the mouthpiece varies for most horn players, the up-and-down placement of the mouthpiece is generally two-thirds on the upper lip and one-third on the lower lip. When playing higher notes, the majority of players exert a small degree of additional pressure on the lips using the mouthpiece. However, this is undesirable from the perspective of both endurance and tone: excessive mouthpiece pressure makes the horn sound forced and harsh and decreases the player's stamina due to the resulting constricted flow of blood to the lips and lip muscles.