The basset clarinet is member of the clarinet family similar to the usual soprano clarinet but longer and with additional keys to enable playing several additional lower notes. Typically a basset clarinet has keywork going to a low (written) C or B, as opposed to the standard clarinet’s E or E♭. The basset clarinet is most commonly a transposing instrument in A, although basset clarinets in C and B♭ and very seldom in G also exist. The similarly named basset horn is also a clarinet with extended lower range, but is in a lower pitch (typically F); the basset horn predates, and undoubtedly inspired, the basset clarinet.

https://en.wikipedia.org/wiki/Basset_clarinet