I gather Mozart wrote his clarinet concerto for a bassett clarinet but that these days it's played on a normal A clarinet.

The bassett clarinet had 4 extra notes, so what happens to these notes when it's played on a contemporary A clarinet? If the notes are rewritten, are they played an octave up, or what?

The standard version as published by B&H et al is a later arrangement by an unknown hand - the actual autograph does not exist so no-one knows what Mozart intended. The version that most people play on a standard A has a lot of awkward corners, phrases that are moving in descending scale/ arpeggio patterns suddenly tip up to a higher octave. These have been restored in the basset clarinet version, carrying on downwards. Other phrases have been transposed fully down an octave. I don't think its a totally convincing restoration but it's still a towering masterpiece whichever you play/listen to.

Gosh I didn't know that. Thank you very much for your explanation.

Hello!

Listen to this programme: http://www.bbc.co.uk/programmes/p00c3r8c

Emma Johnson and Charles Hazelwood discuss this concerto in detail as well as demonstrating.

Quite informative and interesting...

Yes indeed, thanks for posting that link. It was good to listen to the two versions side by side. Lots of good stuff in there.