Concert for Clarinet and Orchestra in A Major, K 622

As there is no autograph for this concerto and as it was published posthumously, it is difficult to understand all of Mozart's intentions. The only relic of this concerto written in Mozart's hand is an excerpt of an earlier rendition of the concerto written for basset horn in G (K. 584b/621b). This excerpt is nearly identical to the corresponding section in the published version for A clarinet. Most likely Mozart originally intended the piece to be written for basset horn, as Anton Stadler was also a virtuoso basset horn player, but eventually was convinced the piece would be more effective for clarinet. However, since several notes throughout the piece go beyond the conventional range of the A clarinet, we can presume it was intended to be played on the basset clarinet, a special clarinet championed by Stadler that had a range down to low (written) C, instead of stopping at (written) E as standard clarinets do.

source: www.wikipedia.org