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It is supposed to be only the 7th degree that gives us the fully diminished 7th chord but when I took a closer look today at the 2nd degree of the harmonic minor scale (Locrian 6) it seems to have an enharmonic equivalent of the a fully diminished 7th chord. The only difference is that the bb7 degree is a 6 in the Locrian 6 scale. So even though you spell this chord differently, isn't it still a full diminished 7 chord?

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Well, a diminished vii evenly divides the 12-note octave into 4 parts. Therefore, WHENEVER you have ANY dim 7 chord, you automatically have another 3. So in C minor:

vii dim7 = b d f a♭

ii dim7 = d f a♭ c♭

iv dim 7 = f a♭ c♭ e♭♭ (or you can call it e♯ g# b d but then it's not really a 4 chord of c)

vi dim 7 = a♭ c♭ e♭♭ g♭♭ ( or g# b d f)

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  • Exactly. All four inversions of a fully-diminished seventh chord are enharmonically equivalent (which is why the spelling is so important). thus, there are only three enharmonically unique such chords.
    – Aaron
    Apr 24, 2021 at 13:48
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The 4 note chord based on the second degree of the harmonic minor scale is not diminished. It's m7♭5. Often referred to as half-diminished. But not fully diminished.

Let's take A harmonic minor. from 2nd note - B, D, F, A. Bm7♭5. To be Bdim., it would need the notes B, D, F, A♭. That Bm7♭5 may also be known as Dm6 in a different inversion.

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  • A harmonic minor from second note also has notes B D F G#.. G# is enharmonic to Ab. I know this is spelt differently but how different is it really?
    – user35708
    Apr 24, 2021 at 10:38
  • Count up in 3rds. B, D, F, A. Diminished, like a lot of other chords, have their intervals each one third apart from its neighbour. There is no Ab in that key anyway. The note numbers for the chord you reference are 2, 4, 6, 8. Not 2, 4, 6, 7.
    – Tim
    Apr 24, 2021 at 10:41
  • Ok let me try this another way... play the notes B D F G#... what do you hear?
    – user35708
    Apr 24, 2021 at 10:42
  • Obviously a fully diminished chord. Would you name Co as C, Eb, Gb, A?
    – Tim
    Apr 24, 2021 at 10:44
  • no but would you stil be Tim if I had a nickname for you? What I am saying is, in practice isn't it the same thing?
    – user35708
    Apr 24, 2021 at 10:50
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There are actually 3 different dim7 chords like the viidim7 in minor built by 4 minor thirds. After 3 semitones they are repeated - if you consider the black and white keys on the piano.

To each tone of the 12 tones you can build a secondary vii7 dim (like there are secondary fifths in chord progressions). Theoretically there are 12 viidim7 - or even more if you regard the equivalent keys like F#/Gb or d#/eb ... etc.

But practically there are only 3 (and their inversions) to learn.

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