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Part of the brass family of instruments, a tuba is both a category of instrument, and a group of instruments within that category. The category tuba includes the baritone, euphonium, and sousaphone, as well as the tuba proper. Numbered among the tubas are the Bb tenor tubas; the F, Eb and C tubas, and the BBb (say “double B-flat”) and CC (say “double C”) contrabass tubas.
Modern tubas are non-transposing instruments, and the C and the BBb are most commonly found in orchestral settings, due to their more comfortable fingering. They often are used to perform older repertoire, originally intended for other brass instruments, such as the ophicleide — a forerunner of the tuba — or the Wagner tuba.
The tuba’s role as a supporting instrument is well-known—so common, in fact, that it is gently parodied in the children’s story “Tubby the Tuba,” in which Tubby finally convinces others that he should be allowed to play a melody instead of only being responsible for the “oom-pahs.” The story of Tubby was written by George Kleinsinger and Paul Tripp in 1941, inspired by the comment of a tubist who had performed one of their pieces.
Composers writing for tuba have to take into account the great amount of breath that is required to play the tuba. Although the tuba player sits still, playing the tuba is a very physical activity, for this reason. Pauses to allow the player to breathe are important. Unlike the other brass instruments, the tuba rarely uses mutes, and when it does, it is usually the straight mute that is required. The mute is awkward, both to insert and the remove, and composers need to allow adequate time for the performer to be able to do it quietly.
Tubas are found in concert bands, orchestras, jazz, Dixieland, brass choirs, and brass quintets. Repertoire for solo tuba includes Concerto in F minor by Ralph Vaughan Williams, Divertimento for Tuba and Piano by Carson P. Cooman, and Enchorial Landscape by Thomas L. Read, and is more common today than heretofore.
Famous tubists include Roger Bobo, James Gourlay, Patrick Harrild, Patrick Sheridan, Sam Pilafian. Other notable tubists are Jon Sass, François Thuillier, Sérgio Carolino, and Harvey Phillips.
From: http://www.wisegeek.com/what-is-a-tuba.htm