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 crookscircular lengths of tubing that could be attached and removed rapidlyfor music modulating to new keys. Since c. 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.