Hi guys, @Pierrot thanks for tagging!
Here are my thoughts on diminished scale/chord.
First, keep in mind that I always try, and suggest students as well, to simplify as much as possible.
I always think of the diminished scale as ONLY whole tone/ half tone, and here’s why:
Let’s start with the ACTUAL diminished chord, here D dim:
Now, when we really try to hear the essence of diminished chord, there are only the following chord tones possible (works the same way as with any chord, the “extra” chord tones after the 7th (9, 11 and 13) will be added as colors, but they are not the “main” tones):
Now, if you look at the extra chord tones of the full diminished chord, you can notice that the added tones are actually an approach notes, half step below to all of the main chord tones:
And, if you look at it this way, here’s your diminished scale!
You might think that this still could work with half/whole tone as well, but the function of a chord behind half/whole is not a true diminished, it works as a dominant chord, just as @Pierrot mentioned:
How I see the diminished chord used over a dominant, comes again together with the II V thinking; D-7 G7 becomes D diminished (so of course G7b9), so the scale I would use is
This way of thinking simplifies diminished in other situations as well, for example a tonic diminished (meaning diminished chord resolving to I), here in the key of C major:
Let me know if you guys have any questions/ideas!
Here are fingerings for diminished thirds (that work for me):