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In a diminished triad, the diminished interval is 1>5. In a full diminished seventh chord, the 1>7 is also diminished. In a half-diminished chord, the 1>5 is diminished.(But one third of it still has a minor third!).

The third is always m3, and that sounds good. But what is the technicality behind 'it must be m3'. Theoretically, one may expect dim.3, but that's not good. I realise it sounds better with m3, but that's m3.

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Richard
    Feb 12 at 17:38
  • Yes, it's a dupe, which either I missed, or it didn't show up. My apologies.
    – Tim
    Feb 12 at 17:42
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It seems there are two ways of thinking about this.

  • chords are arbitrary intervals above a root
  • chords are the diatonic intervals above a root

It isn't purely one or the other, but I think the primary foundation is diatonic with some chromatic modification.

Rather than a diminished triad being arbitrarily a minor third and diminished fifth above a root, it's the diatonic triad rooted on the leading tone.

I think this accounts for most common practice harmony. Even a chromatic chord like a French sixth chord is just an inversion of diatonic iiø7 with the chord's third raised. In other words, it's just a diatonic chord iiø7, with one tone altered.

Diminished triads has a minor third, because the diatonic viio has a minor third. That diatonic chord is the basis for identifying a functional diminished chord.

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  • I like the logic behind this. Not that I expect theory to be logical - although isn't that what any theory ought to be based on? So, purely diatonic? That accounts for the triad.
    – Tim
    Feb 12 at 17:30
  • Accepted your answer. Forgot about the dupe, which ironically, i commented on profusely. Don't get old - it's not a lot of fun.
    – Tim
    Feb 12 at 18:42
  • Youth is wasted on the young! Ha ha! Feb 12 at 18:59

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