Quick Theory Question

[Relayer]

Okay guys,

If a song is based around the notes of an A diminished (half/whole) scale. What key signature would i use to notate it?

Thanks in advance!

[Joe Pass Jr]

My first thought would be they key of D. Thats just based on Adim chord being a nice sub for A7 which would generally resolve to D. However if you could give us some more info on the chord progressions involved or post the actuall melody/head/chorus of the song it might make things easier.

[JonR]

A blank key signature (ie none).
This is because the diminished scale has no tonal centre.
You can call it "A half-whole diminished" - but that doesn't mean A is the root, not functionally anyhow.

This scale would normally be used (in jazz) on an A7b9 chord, resolving probably to Dm, or maybe D major. IOW, it's a tension. It has an A root in that context - and in a D minor context you would use a D minor key sig (with appropriate accidentals for the chord) - but I'm guessing you are not using the key of D minor .
(BTW, Joe Pass Jr is not right that Adim substitutes for A7. It would be Bbdim7, or C#dim7, because that contains all the chord tones, apart from the root.)

If you are really composing something entirely in a diminished scale, I would use no key sig. OK, this reads as a C major/A minor key sig - but that's the most sensible starting point.
(Blank key sigs are also used for modal jazz where the modes constantly change.)

[Relayer]

Thanks a lot guys!

I was thinking that a blank key signature probably makes the most sense too, but i was wondering if there was some correct convention that i didn't know about!

Cheers!

[Joe Pass Jr]

thanks for the correction there JonR. I was infact thinking Gdim7 at the time, or ofcourse Bb , C#/ Db as you mentioned.