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A descendant of the bugle, like the mellophone, the euphonium is one of the larger member of the tuba group of the brass family of instruments. Other members of the tuba group are the baritone, sousaphone, and the tuba itself.

Like the tuba, the orchestral baritone horn, and the alto or tenor horn — it is, confusingly, called by both names — the bell of the modern euphonium points up. But this was not always the case. From the 1880s to the 1960s, a double-belled euphonium was made to try to capture the attributes of both trombone and euphonium in one instrument. In some of these hybrids, both bells are angled between upright and forward. In others, one bell points up and one points forward.

Of all the instruments in the tuba group, the euphonium looks most like a baritone horn, and in fact, the instruments are remarkably similar. Both instruments are in Bb, and they have a similar range. The two are used commonly in bands, but more rarely found in orchestral music, and each instrument is often substituted for the other. There actually has been a certain amount of confusion surrounding composers’ intentions with regard to which instrument should be played, both because of the frequency of substitution and because the euphonium is also called the tenor tuba in English and is known as Baryton in German and baritono in Italian.

Nevertheless, the baritone and euphonium differ in some important ways. Besides a different number of valves — the euphonium usually has four, while the baritone has three — variations in the bore and the bell are the chief differences, with the result being that the sound of the baritone is “brighter,” while the sound of the other instrument is described as “darker.” The difference in the number of valves means that the fingering is different as well. In addition, there are a number of brass quartets that specify two cornets, horn or alto horn, and euphonium.

Also, perhaps related to the difference in sound, there is more orchestral repertoire for euphonium, though in some instances it is a case of traditional substitution rather than the composer’s original designation. Today, the instrument is characteristically heard in the solo in “Bydlo” in the Maurice Ravel arrangement of Modest Musorgsky’s Pictures at an Exhibition, in “Mars” in Gustav Holst’s work The Planets, Shostakovich’s The Golden Age, and in Richard Strauss’s Don Quixote. Allen Hovhaness’s Diran is for euphonium and string orchestra. Some pieces that have clearly been written for this instrument include a number of works of Havergal Brian.

Pieces for solo euphonium and band are common. Examples include Euphonium Concertos by Joseph Horovitz, Derek Bourgeois, John Golland, and Philip Sparke, as well as Bourgeois’ Euphoria for Euphonium and Brass Band. Philip Sparke has also written Fantasy for Euphonium. Noted players include Simone Mantia (1873–1951) and his teacher, Joseph Raffayola. Contemporary players include Wendy Picton, Nick Childs, and Steven Mead.

From: http://www.wisegeek.com/what-is-a-euphonium.htm