6

Checking out something in the ABRSM manual of scales and arpeggios, I note that those 'formed of the chords of the dominant seventh' start and finish on the dominant note (In C, goes G-G). That makes sense.

Those 'formed of the chord of the diminished seventh' start and finish on the leading note (In C, goes B-B). Surely that's not 'of the dim.7'. The dim7 of C is Bbb, I thought. How is this explained?

5

I think the important distinction here is that between "the diminished-seventh chord on C'' and "the diminished-seventh chord in the key of C."

You're correct: the diminished-seventh chord on C has B♭♭. But the diminished-seventh chord in the key of C is going to be b°7: B D F A♭. This is because, when one considers the diatonic seventh chord, the only fully diminished seventh chord is that built on the leading tone.

But note that this suggests either C minor or the joint C major/minor (that is, modal mixture is included), because the chord build on the leading tone in a major key is actually half-diminished: B D F A, or bø7.

  • 1
    The notes are indeed B D F Ab. Not even diatonic. – Tim Apr 6 '18 at 14:00
  • 1
    Modal mixture, baby. Gotta have that angst! – Richard Apr 6 '18 at 14:03
1

The dominant 7th-shape chord that can be built using the notes of a C major scale is G7. There is no diminished 7th chord that can be made exclusively from those notes, but the diminished TRIAD that can is Bdim. So I guess that's why they lumped Bdim7 into the 'C major' group.

It's arguable that the most commonly-used dim7 chord in a piece in C major is C#dim7, in the ubiquitous C, C#dim7, Dm7, G7 sequence.

  • Yes, but a little spurious for an organisation such as that? Mind you, they only allowed natural minor about five years ago! – Tim Apr 6 '18 at 15:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.