The basset clarinet is a clarinet 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, as opposed to the standard clarinet's E or E♭ (both written), and is most commonly a transposing instrument in A, although basset clarinets in C and B♭ also exist, and Stephen Fox makes a "G basset clarinet/basset horn". 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.
The basset clarinet was most notably associated with the clarinet virtuoso Anton Stadler (1753-1812), a contemporary and good friend of Mozart. Mozart wrote his Clarinet Quintet in A major, K.581 and Clarinet Concerto in A Major, K.622 for this instrument; the concerto is partly based on an earlier fragment of a Concerto for Basset Horn in G, K.584b. There is an aria in Act I of Mozart's last opera, La clemenza di Tito, (the mezzo-soprano Sesto sings Parto, ma tu ben mio) which features a basset clarinet obbligato.
As noted above, a basset clarinet is an A clarinet with an extension of a major third down. It is in fact related to the basset horn in F or G. Because Mozart's clarinet concerto is so important, the basset clarinet is quite an interesting instrument in spite of its small applicability. For the concerto the extension must be chromatic and the shape of the Viennese basset horn is not suitable for this. It has long been unclear how this instrument might have looked.
In a library in Riga in 1992 programmes were found of concerts which Anton Stadler played there in 1784. Two of those programmes show an engraving of Stadler's instrument.
Franz Xaver Süßmayr also wrote a concerto movement for basset clarinet.
The term "basset clarinet" was in use by 1796, though it may originally have referred to the basset horn.
Despite Stadler's advocacy the instrument did not become a regular member of the orchestra. During the 19th and early 20th centuries only a few basset clarinets were produced, for performances of Mozart pieces, and no further music was written for the instrument. However, beginning in the mid 20th century, interest in performing on original instruments prompted the basset clarinet's revival. A few modern composers, among them Bill Sweeney, Harrison Birtwistle, Alan Ray Hacker, Hannes Pohlit and Franklin Stover, have written works featuring basset clarinet; Joan Tower's 1988 clarinet concerto is written to be played on either basset or standard clarinet. Many clarinet makers now produce basset clarinets, or extended lower joints which will convert a standard clarinet to a basset clarinet.
Classical clarinetists who have recorded albums using basset clarinet include Martin Fröst, Sharon Kam, Kari Kriikku, Colin Lawson, Sabine Meyer, Antony Pay and David Shifrin. The German clarinetist Theo Jörgensmann plays free jazz on a basset clarinet as does Los Angeles based performer Vinny Golia (who also uses the Basset Horn in his music). The British clarinetist Thea King recorded both Mozart's Quintet and Concerto on the basset clarinet for Hyperion Records, coupled together on one CD. Michael Collins, who studied with Thea King, has recorded the Mozart concerto playing a basset clarinet (Deutsche Grammophon, along with a transcription for clarinet of Beethoven's Violin Concerto). With the North Carolina Symphony on April 10, 2008 Collins premiered Elena Kats-Chernin's "Ornamental Air", in the form of a concerto for basset clarinet. Another British player Joy Farrall has also recorded Mozart's concerto and quintet (BMG and Meridian) using a basset clarinet, alongside the 'Kegelstatt' trio for clarinet, viola and piano. On period instruments, Jane Booth has recorded the Mozart quintet with the Eybler Quartet (Analekta, 2010). Working across practice and theory, Colin Lawson's celebrated recording of Mozart's Clarinet Concerto K.622 with the Hanover Band for Nimbus, released in 1990, complements his Cambridge Handbook to Mozart's Clarinet Concerto, published in 1996.