Orchestral and military brass instrument , a valved circular horn with a wide bell.
It is normally a transposing instrument (its music written in a different tone than its actual sound) in F. It has a wide bore and three (sometimes four) rotary valves; its conical mouthpiece produces a mellower tone than the cup-shaped mouthpieces of other brass instruments. Horns long relied on separable crooks
circular lengths of tubing that could be attached and removed rapidly
for music modulating to new keys. Since с 1900 the standard horn has been a "double" instrument, with built-in crooks in F and B-flat that can be selected rapidly by means of a thumb valve. The modern symphony orchestra usually includes four horns. Though difficult to play and prone to producing conspicuous errors, its tone is widely admired.