In the days when I tuned my harpsichord I became deeply respectful of the singular problem posed by the black keys we call F# and B-flat. Properly tuned in a version of mean-tone temperament, they sound absolutely horrible together.
Even on a modern piano tuned to equal temperament, this combination of keys will inevitably sound like a major third, never a plausible diminished fourth.
That probably explains why Mozart stayed away from the key of G minor on the piano, as did Beethoven, with some notable exceptions: The Piano Quartet K. 478, for example, on which my book is forthcoming.
Haydn, on the other hand, has written in that key, and has taken on the problem in highly creative ways, bless his heart. His trick is to use the ordinarily F# key as a G-flat, thus relishing the major third produced by those particular black keys.
It makes for remarkable tonal passion.
