Writing Key Signatures

I've never analyzed Bach, I play his tocattas, fugues, preludes, and ect. all the time, but I guess I've only been subjected to his 'simpler' works. :happy:

Try some of the fugues from the WTC, look at the harmony, that's harmonic ADD if I ever saw it, and it still sounds awesome :happy:

This is a kind of weird discussion...

Key signatures exist because they imply some kind of tonality, be it modal or major/minor. Even if, like Bartok, you invent a key signature (like A flat only) it implies some kind of hierarchical system, certain pitches having more importance than others. So if you're writing in a free non-tonal then it just doesn't make sense to use a key signature. Likewise if you're writing music where a certain tonality is briefly implied but does not define the pitch structure of the whole piece it's a good idea not to use a new key signature.

And of course in tonal music there are distant modulations, Ravels' piano concerto in G - whose second movement is written in E, briefly moves into D sharp major - an extremely awkward key to read and understand etc. But he doesn't change the key signature and sticks to sharps because it ultimately refelects the tonality of the movement. Changing key signature is intended to show a fundamental change in the tonal structure of a piece.

But if you're writing atonally, then KEY signature just doesn't come into it - the piece is not in a key and it can't be expected to apply.