For example in the C diminished scale: do I use a ton of accidentals or should I come up with some weird key signature that has something like a D# and use the C# as an accidental on the line of C?
The diminished scale has 8 pitches, but in our Western notational tradition, we have 7 note names. As such, when you're writing the scale, it's vital that you use all 7 notes names at least once, and only one note name twice.
In other words,
is not ideal because it repeats both the
C and the
A note names, and in the process omits B, destroying the very nature of the scale. Better would be
which replaces the second version of
A with a form of
The logic involved here is the same reason we don't spell the C-major scale as
As for accidentals, composers have different practices:
- If a diminished (=octatonic) portion is based on some tonic, often it will be spelled as similar to that overall tonality as possible. If it's loosely based in B♭ major, for instance, they'll use B♭s as opposed to the less-common tonic of A♯.
- Some composers just plain always use flats, no matter what; others always use sharps.
- Others will switch around depending on the circumstance and what instrument (especially transposing instrument) is playing at the time.
And in actual musical situations, you tend to want to follow the general rule that a chromatic pitch resolves in the direction of its accidental. In other words, instead of notating
D♭ moving up to
D♮, it's wisest to use fewer accidentals and notate that first pitch as
C♯ (a raised pitch) will want to resolve up (the direction of its accidental).
Regarding key signatures, I've seen a few practices:
- Just use a blank key signature and add in all accidentals. In my opinion, this is most common.
- If there's a central tonic in your octatonic section, you can use the key signature for that tonic and include any necessary accidentals when they appear. Which form of the octatonic scale you use will determine if it's cleaner to use the major or minor form of the key signature.
- Or you could just invent an odd key signature, like one with F♯, C♯, D♯, and A♯ (notice there's no G♯). Even better, you can go the Bartók route and have different key signatures in each hand, which combined create an octatonic collection!
I'd suggest you use the key signature of the major scale with the same starting note and then add accidentals. In the particular case you mention, C Major has no sharps or flats in its key signature.
There are two diminished scales starting on a C natural, depending on whether the first interval is a Tone or a Semitone.
C C# D# E F# G A A# C
C D D# E# F# G# A B C
If you just roll your own key signature which has a non-standard collection of sharps or flats (or a mixture of sharps and flats!) you will confuse the reader. The idea should be to unambiguously express your intent.
My composition professors eschewed key signatures in situations like this. One suggested to write in the accidentals before each note, using the most common sharps and flats (the first three of each.) The other suggested to use sharps or flats depending on what was happening in the music, especially when writing for strings. He would say that if the line is ascending to use sharps, and if the line is descending to use flats. This is a bit oversimplified, but captures the essence of what he was saying.