Music man

OK, here's a question for you - in what key was Mozart's Symphony No. 40 written? G minor, right?  Well, yes and no.  It was certainly written with a key signature of two flats, which is the right key signature for G minor, and if you hear it played today and pick out the keynotes on the piano, it will come out as G minor. So why yes and no?  It's all to do with pitch.  Just how do you decide what note sounds like what? And throughout history there have been different approaches. The A above middle C, which orchestras traditionally tune to, has been officially set at 440 Hz since 1939, but it wasn't always so, and even today it is not universally adhered to (the New York Phil for example tunes to 442).  In particular, in Mozart's time, this A was set at 424.2, which is pretty well a semi-tone lower than today, so you could argue that Symphony 40 as originally written would have sounded to modern ears as being in something closer to F# minor.