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The tuba is a type of brass instrument from the family of instruments which includes the euphonium, the sousaphone, the lower saxhorn and others simply called “tuba.” The exact type of tuba one would hear in an orchestra depends on the country. The contrabass tuba in C has become the standard in the United States. The tuba replaced its prototype, the ophicleide, as the bass member of the brass section in the 1830s. The Tuba’s conical bore and long tube give it a smooth, rich tone more like the horn than the trumpet or trombone. Wagner developed his own brass instrument, the tubin (now called a “Wagner tuba”), which is not wholly related. He used the tuba extensively, notably for the dragon’s music in Ring cycle. The tuba has been taken up as a solo instrument by jazz musicians and avant-garde composers.
Source: The Norton/Grove Concise Encyclopedia of Music, edited by Stanley Sadie. London: Macmillan Press Ltd.,  ISBN 0-333-43236-3