G flat major vs f sharp major

[ravelfan07]

When does one know to use one or the other?
Similar to other examples like d flat maj vs C sharp maj, c flat major vs B major, etc

[psipsi8]

Depends what key gets transposed from, e.g. I to V where I = B major then V will be F-sharp major. When transposing I to IV and I is D-flat major, IV will be G-flat major.
Now F-sharp major as tonic, it's less usual than G-flat major but can be found, e.g. Chopin Impromptu no.2. This piece contains a lot of modulation and resulting double-sharps, for example when it modulates to V (C# major) temporarily, e.g. the chord II7 in the key of C# major which is D# major(!) as shown in the first bar of the attached excerpt. Whereas if the piece were nominally in G-flat major, they would be written merely as naturals. Although, i haven't analyzed all of the piece, there are sections where the modulation reduces the number of accidentals.

[transitional]

In Hungarian Rhapsody 2, the instance of F-sharp Major is quite obvious because it switches from F-sharp Minor.

It seems like in the Well-Tempered Clavier, the fashion of chromatically ascending scales allows awkward enharmonic key signatures if they don't go overboard. For example, there is F-sharp Major and C-sharp Major, but you wouldn't come across G-sharp Major here or anywhere else.

Just some thoughts. I'm not a big expert on the matter myself, but it is most commonly for the purpose of modulating smoothly. This is only enharmonically shifted (like A-flat Major instead of G-sharp Major) for large chunks that would get awkward with that heavy amount of accidentals, from my understanding.

[lelle]

Sometimes it's just that the composer felt that F sharp suited the piece better than G sharp or vice versa, for whatever pyschological reason.

Otherwise, it would typically have to do with the harmonic relationship to surrounding pieces or passages. Let's say I wrote a sonata in B major and wanted the second movement in F sharp major / G flat major, F sharp would be the better choice since it's the dominant of B major. Choosing G flat would make less sense.

[medtnerfan]

Readability and modulations. Personally I find flats easier to read compared to sharps, however if the piece is temporarily modulating to keys with less sharps than flats in their enharmonic conterpart, then I might use the key signature with sharps. I actually recently put out an original composition where it starts in C-sharp minor and ends in the parallel major, but I put the end in the key of D-flat Major instead of C-sharp Major. My reasoning was because it was easier to read.
Link for composition if you're interested (Sonata for left hand alone):