Key signatures in classical works

[Skip Skipson]

Very interesting. I'm aware that there is such a thing as equal temperament, and that things were different before its advent, but that's about all I know about it.

Answers I've had so far to "Why are classical pieces often referred to by their key signature?", and its corollary "Why did a composer choose a certain key for a certain work?":

1. To differentiate them from other pieces by the same composer
   
2. To accommodate the notes that can be played by the instruments that are intended to play the piece (as I understand it, not every instrument can play every note)
   
3. That the key can make the piece easier or more difficult to play
   
4. That certain keys are arguably thought to create certain moods
   
5. That the "mood effect" may have been different and more real before the advent of equal temperament
   

I'm still not certain that we've gotten to the bottom of it, though.