Originally Posted By: jford
Billy -

It's a matter of the music theory. It's not so much an issue of equivalence. If I'm playing in the key of G-flat, I am not playing in the key of F-sharp (which, at least on a piano, is tone equivalent to G-flat), and so if I go up a 4th for the next chord, I expect that chord to be a C-flat, not a B-natural (which is what I expect were I in the key of F-sharp.

Notation-wise, the G-flat chord would have notes on G, B, and D lines (in treble clef), but with the key signature consisting of 6-flats so all three notes are flatted. My brain has already acknowledged the key signature, so going up to the 4th would be confusing to all of a sudden see a B chord with accidentals on the notes B, D-sharp, and F-sharp, rather than no accidentals on the notes C-E-G (but all flatted by the existing key signature). B is not the 4th of G-flat; C-flat is. Likewise, C-flat is not the 4th of F-sharp; B is.

That's what's being asked for.

Great response John, precise and exact (no surprise).
Just sight-read the score of Moonlight Sonata (Opus 27, Beethoven). Written in the key of C# minor, it includes significant intonation, with notes throughout such as B# instead of C natural, because the written note of C would not correctly fit with a melody written with a key signature of C# minor. Other double sharps persist throughout the movement. It's a reasonably simple song to listen to, but to play requires lots of concentration. Accidentals throughout. Hence this discussion.

Trevor

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