From Academic Kids
A serpent is a bass wind instrument with a mouthpiece like a brass instrument but side holes like a woodwind instrument. It is a long cone bent into a snakelike shape, hence the name. The serpent is closely related to the cornett, although it is not part of the cornett family, due to the absence of a thumb hole. It is generally made out of wood, with walnut being a particularly popular choice. The outside is covered with dark brown or black leather. Despite wooden construction and the fact that it has fingerholes rather than valves, it is usually classed as a brass, rather than a woodwind, instrument. The Hornbostel-Sachs scheme of musical instrument classification places it alongside trumpets.
On early models, the fingerholes were keyless, like on a recorder. Later models, however, add keys, as on a clarinet, although the keys were for additional holes (out of reach of the fingers), while the original holes remained uncovered & unkeyed. The range varies according to the instrument and the player, but typically covers a range from two octaves below middle C to at least a half octave above middle C.
The instrument was first used to strengthen the sound of choirs in plainchant. Around the middle of the 18th century, it began to be used in military bands and orchestras, but was replaced in the 19th century by a fully keyed brass instrument, the ophicleide, and later on by valved bass brass instruments such as the euphonium and tuba. Since then, it has hardly been used at all, although many original models still survive, and it is sometimes played as part of historically authentic performances.
A variation on the serpent was the bass horn, which is essentially the same, but is simpler in shape, consisting of a tube folded back on itself (rather like the modern bassoon), rather than the curvy shape of the original instrument. Another common variation is the so-called "Russian Bassoon", which is neither Russian nor a bassoon.