> The idea is that each note of the scale should fall on a successive line or space on the staff, and the "convention" of having keys be only sharps or flats falls out from that (hm, why? I've never thought about that).
The second part is really the same as the first. You want to be able to spell each diatonic alphabetically in order without skipping any letters of the alphabet. The B major scale is
B C# D# E F# G# A# B
If you tried spelling that with flats, you'd get
B Db Eb E Gb Ab Bb B
The skipped C and F, and the double E and B, is just nasty. The same would happen if you tried spelling the scale with a combination of flats and sharps.
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.
> 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.
The second part is really the same as the first. You want to be able to spell each diatonic alphabetically in order without skipping any letters of the alphabet. The B major scale is
If you tried spelling that with flats, you'd get The skipped C and F, and the double E and B, is just nasty. The same would happen if you tried spelling the scale with a combination of flats and sharps.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.