The Basset Horn is a curious member of the clarinet family. Traditionally, it is a small bore (roughly equal to the bore size of the B-flat and A Clarinets) instrument pitched in F (a fifth lower than written). Again, traditionally, the Basset Horn was to be played by B-flat/A Clarinetists with the same mouthpiece used on those instruments. However, in practicality, most modern instruments do not conform to this ideal. The bore has been enlarged, and in some cases is the exact same as to be found in the Alto Clarinet, so that in reality, the Basset Horn is now simply and Alto Clarinet in F.
Unlike other clarinets, the Basset Horn always possesses a written low C (here sounding F at the bottom of the bass clef). In other clarinets that only occasionally descend to the low C; these notes are often referred to as the “Basset” notes.
It was said to be one of the favorite instruments of Mozart, who scored for it in his later operas, the Requiem, and his music for wind ensembles. Beethoven, Mendelssohn, and Richard Strauss have all used the instrument. Strauss, in particular, used it quite extensively.
For a comparison between the F Alto and the E-flat Alto, read this article.
Beethoven’s Horn Sonata on Basset Horn
Mozart Duet for two Basset Horns
Excerpt from Mendelssohn’s Konzertstuck
The Adagio from Mozart’s Gran Partita. Listen for the interplay between 1st Oboe, 1st Clarinet, and 1st Basset Horn.