From Academic Kids
The euphonium is a valved brass instrument, the tenor member of the tuba family. The appearance of a euphonium is very similar to that of a tuba, and many people not familiar with the instrument sometimes incorrectly call it a tuba. A person who plays euphonium is called a euphoniumist.
Like the tuba, the euphonium has a bore that constantly increases in size from the valve section to the bell - this is called "conical bore", as distinguished from "cylindrical bore" instruments, which maintain a constant tubing diameter for a greater portion of the instrument's total length. Thus, even though the euphonium plays in the same range of pitches as the trombone (with a cylidrical bore), it has a more mellow, "rounder" sound than the "brassier" sound of a trombone. This is similar to the difference between the cornet and the trumpet.
Typically, a euphonium has four valves. Each valve increases the length of the entire instrument when pressed down, according to the length of the tuning slide attached to it. The order of length from shortest to longest is 2, 1, 3, 4. The 4th valve is about the same length as valves 1 and 3 combined. The 4th valve exists because it is slightly more in tune in some contexts than valves 1 and 3 combined. Valve 4 also allows for a player to play in the lower register, by adding more tubing than the usual 3-valve instrument's limit. Some euphoniums also include 'compensating systems', consisting of extra tubing in the fourth valve. These both improve the stability of the instrument's sound in the lower ranges and allow the player to play in those ranges using more conventional fingerings.
A similar instrument is the American-style euphonium, which is often (incorrectly) called a baritone horn. This design sometimes features a curved bell section that points forward, and is sometimes made with the valves on the front of the instrument. It is as conical in design as the euphonium shown here, but usually has a slightly smaller bore (although the bore is much larger than the British-style baritone horn). Other similar instruments include the British baritone horn (which, being a saxhorn, is more cylindrical and trombone-like than the American baritone), and the German baryton and tenorhorn. All trace their descent to the serpent. A unique American creation was the double-bell euphonium, which featured a second, smaller bell and an extra valve allowing the player to use that bell instead of the main bell. The second bell had a more trombone-like sound. It was used in the early part of the 20th century but fell out of favor after that. No current manufacturer makes a double-bell euphonium.
The euphonium is most commonly found in wind bands of various kinds, such as concert bands, brass bands, marching bands, and military bands, where it is frequently featured as a solo instrument. It is predominant in many marches, such as those written by John Philip Sousa. It is also often used today in brass ensembles/choirs. It is not traditionally an orchestral instrument and thus is not found in modern symphony orchestras. Only for certain works where the composer specifically requested it is a euphonium (or "tenor tuba") used in orchestras (such as in Holst's Planets Suite). The name "euphonium" comes from the Greek word "euphonion", meaning "beautiful-sounding" or "sweet-voiced".