The Euphonium is part of the family of brass instruments. It is sometimes confused with the baritone horn. The Euphonium and the Baritone differ in that the bore size of the Baritone horn is typically smaller than that of the Euphonium, (leading to a “darker” tone from the Euphonium and a brighter sound from the baritone horn) and the baritone is primarily cylindrical bore, whereas the Euphonium is predominantly conical bore. The two instruments are easily interchangeable to the player, with some modification of breath and embouchure, since the two have identical range and essentially identical fingering. The cylindrical baritone offers a brighter sound and the conical Euphonium offers a mellower sound.
The so-called American Baritone, featuring three valves on the front of the instrument and a curved, forward-pointing bell, was dominant in American school bands throughout most of the 20th century, its weight, shape and configuration conforming to the needs of the marching band. While this instrument is in reality a conical-cylindrical bore hybrid, neither fully Euphonium nor baritone, it was almost universally labeled a “Baritone” by both band directors and composers, thus contributing to the confusion of terminology in the United States.