First utilized by the military, the euphonium, or tenor tuba, is one of the brass family's most versatile instruments. It has a rich, warm tone, and often fills the niche of the cello in modern wind ensembles.
The euphonium is a conical instrument (in contrast with the baritone, its cylindrical cousin), which gives it a darker tone and wider range of timbres. The most basic euphoniums have three pistons, or valves, but one worth its salt will have four for the low notes (it is a tuba, after all). The best of these have extra tubing associated with the fourth valve for the lowest notes, rerouting air through extra tubing to compensate for sharpness. (Bravo, technology!) Since the essential construction of a brass instrument is a tube into which one buzzes one's lips, creating different levels of air vibration, one needs a mouthpiece to plop into a bore in the horn. The tangled mass of tubing through which the air takes its mystical journey ends in a bell, or, in some older, more interesting models, two bells (the second one is activated with the fourth valve). This is pretty much so you can hear the thing, although the bell(s) often can make it look purty.
Because the euphonium has a wonderful range of tones and is valve-operated (as opposed to the trombone), it is widely used for more intricate or melodic phrases in music. Most composers are content with using the euphonium as part a solid foundation (low brass' specialty) for their works. Consequently, there are not a great deal of solo pieces for euphonium. Gustav Holst is one of the few composers that truly features the euphonium; one of the main themes in Jupiter: The Bringer of Jollity (a part of his suite, The Planets) is introduced in a euphonium solo, and movements in his First Suite in E-flat and Second Suite for Military Band in F also prominently feature the euphonium.
The proper noun to use to refer to one who plays the euphonium is tubist
is the precursor to the euphonium, and saxhorns are still sold in Europe today. In fact, the "trumpeter's Bible" written by Jean-Baptiste Arban was originally composed of studies for both the cornet
(a conical version of the trumpet
) and the saxhorn.