brass instrument, in music, any wind instrument—usually of brass or other metal but formerly of wood or horn—in which the vibration of the player’s lips against a cup- or funnel-shaped mouthpiece causes the initial vibration of an air column. A more precise term is lip-vibrated instrument. Ethnologists frequently refer to any instrument of this class as a trumpet; but when they are made of or derived from animal horns, they are also often known as horns. Typical brass instruments in a Western orchestra are the trumpet, trombone, French horn, and tuba (qq.v.).

A lip-vibrated instrument consisting of a cylindrical or conical tube produces only a fundamental note and, when vigorously overblown, its natural harmonic series (as, for the fundamental note C: c–g–c′–e′–g′–b♭′ [approximate pitch]–c″–d″–e″, etc.). Most modern brass instruments are provided with valves or slides that alter the length of the tube. This gives the players several fundamentals, each with its own harmonic series, thus making available a full chromatic (12-note) scale. Brass instruments, like all wind instruments, are classified as aerophones.