I do not understand the key notation for the clarinet in an orchestral score I am looking at. This is an extract from the Andante of the Ravel concerto in G:
The clarinet has a different key signature than the rest, and though the score uses a treble clef, what the clarinet plays is actually a third below. Surely this has something to do with the fact that the score says "Clar. in la", i.e. something like "play an A when you see a C", but I am confused where that notation comes from and its exact purpose. Why not writing an A if an A needs to be played?