> As for diatonic scales that start on an accidental, as far as I know you can use either the flat or the sharp notation. They are enharmonic equivalents. I know there are often traditional preferences, simply to choose the spelling that leaves fewer accidentals in the key signature. For instance, I think that Db major is more common than the equivalent C# major because the former has 5 flats while the latter has 7 sharps.
Yup, that's true, though the choice of enharmonic keys will depend on context: e.g. the V7/V in a sharp key will be spelled in another sharp key even if more awkward, with double-sharps etc. Also, to get pedantic there are no "accidentals in the key signature" by definition.
> Also, to get pedantic there are no "accidentals in the key signature" by definition.
Yeah, I tried to figure out a better term for that, but I'm not aware of one. Of course, I meant notes that are accidentals in C major, i.e. the black keys on a piano.
Yup, that's true, though the choice of enharmonic keys will depend on context: e.g. the V7/V in a sharp key will be spelled in another sharp key even if more awkward, with double-sharps etc. Also, to get pedantic there are no "accidentals in the key signature" by definition.