The baritone has a long history. It all started in the early 18th century with an instrument called the serpent. This snake-like tube was made of either wood, brass or silver, and its tuning wasn’t good. It had six finger holes (valves wern’t ivented yet) in the beginning, but later more were added. It was used in military bands as a marching bass, but it could also be found supporting the bass part in church choirs. Players of the serpent needed to be extraordinary musicians, because when put in less talented hands it sounded horrible.
The serpent had an undistinguished life, although some people still play it today, and was replaced in 1821 by a brass instrument, produced by a French company called Halary, that was “a combination of the modern bassoon and baritone saxophone” called the ophicleide. The ophicleide used keys (instead of the finger holes of the serpent). It became popular and was made in several sizes and keys, but by the end of the century, it had almost disappeared.
Around 1815, valves were invented by Heinrick Stolzel and Friedrick Blushmel. Valves improved intonation and pitch, and made almost all modern brass instruments possible. At that time, the technology was new, and it was not until 1823 that a horn similar to the euphonium was used.
The 1860s and 1870s were a major time of improvement for the euphonium, beginning when Professor Phasey of Kneller Hall, England, enlarged the bore of the instrument. Soon after that, over the shoulder euphoniums began to appear, and in 1874, the compensating valve system, invented by David Blaikley, made pitch better. The 1880s brought even more variety to the baritone/euphonium scene with the addition of the large bore Kaiserbariton, and the famous double-belled euphonium of Meredith Wilsons The Music Man.
Not much happened in the development of the modern euphonium between 1888 and 1921, because people had decided that there was enough variety and there was no longer a need for new instruments. Baritones were taken out of wind bands and essentially limited to brass bands. Euphoniums however, became standard equipment for any military, marching, or concert band.