The basset-horn is a musical instrument, a member of the clarinet family.
Like the clarinet, the instrument is a wind instrument with a single reed and a cylindrical bore. However, the basset-horn is larger, has a bend near the mouthpiece rather than an entirely straight body (older instruments are typically curved or bent in the middle) and while the clarinet is typically a transposing instrument in B flat (meaning a written C sounds as a B flat), the basset-horn is typically in F and has a range extending below the clarinet to the F at the bottom of the bass clef. Its timbre is similar to the clarinet's, but darker and less brilliant.
Little classical music for the instrument is remembered today, although a number of composers of the Classical period wrote for it and Anton Stadler played it. Wolfgang Amadeus Mozart is the most notable example, including it in his Masonic Funeral Music, Requiem and several operas and chamber works. His Clarinet Concerto in A Major, K622 also started life as a concerto for the basset-horn. (In Mozart's day, the instrument was known as the bass clarinet, though in modern usage this indicates a different instrument.)
In the 19th century, Felix Mendelssohn wrote two pieces for the basset-horn, clarinet and piano (opus 113 and 114), but the instrument was largely abandoned until Richard Strauss took it up once more in his opera Elektra and several later works. In the 20th century Karlheinz Stockhausen has written for it, giving it a prominent place in his cycle of operas Licht and other pieces. On the whole, however, the instrument remains little-used.
The Italian name for the instrument, corno di bassetto, was used by George Bernard Shaw as a pseudonym when writing music criticism.
To confuse matters, not only is it not a horn; it was reputedly invented by a fellow named Horn.
The basset-clarinet is a modern name for a clarinet adapted to play the low notes of the basset-horn. Mozart's Clarinet Concerto was written for this instrument.