So a secondary IV - V is ok but a secondary IV alone or in sequence is not? I guess I could get on board with that. I’m not saying I won’t still abuse the notation, but I’ll be thinking about this while I do it! I also still think secondary options should be unrestricted, both for extended tonicizations and to access the various cadences idiosyncratic to each mode.
My ideal Hookpad would let you enter chords in whatever way you like, whether a secondary from a specific mode, raising or lowering a whole chord by any interval, negating a chord, or raising or lowering any tone by any interval, including its root, third, and seventh, as well as allow stacking of operations. It would then suggest a list of equivalent simplified names you could choose to either replace the convoluted way you found the chord, or display it on a second (or third) line as an alternative name that highlights different harmonic perspectives or modulation opportunities. This would let you easily enter harmonic relatives, extended tonicizations, planing/side slipping, various forms of diminished chords, etc without having to think up it’s canonical name or click on 100 different things, and the app will do the heavy lifting, reducing it down like one might an algebraic equation.
With Head Over Heels, I think I was just pointing out that modal mixture doesn’t always work to describe these sorts of progressions. If the stack of plagals really is incidental, I don’t think harmonic relations fully explain them either, since as you point out this one does have a well defined key center. Maybe its just because we tend to hear the first chord as the tonic, but I kind of like your retrograde progression explanation too. Also relating harmonic momentum to the number of tones that change - I’d never considered that before but it makes a lot of sense, in fact it seems kinda obvious now that you’ve pointed it out!
I learned of harmonic relativity here, where he calls it chord relationships. He also provides a couple tables where he attempts to describe each interval for major and minor chords. Even though both Berthume and Berkemeier say each one is subjective, their descriptions still seem to agree, and Berkemeier’s extended list also seems about right to me. I’d love to see a list that includes augmented and diminished chords, and I’m curious if anyone has explored how sevenths combine – whether it is a union or intersection of emotions, whether the upper or lower triad is more significant, or if it is even more nuanced than that.
As for dominant substitutes, I think iii7 totally works. It’s just a Vadd6 with the 6 in the bass. It just isn’t as useful since it shares so many tones with its target, and I probably wouldn’t use it myself with so many better options. From what I know of negative harmony (pretty much only this), dominant chords must have at minimum one unstable tone and one leading tone, and the presence of a modal tone indicates a tonic chord. iii7 then is both dominant and tonic, which I think describes it pretty well.
Here’s a list of the rest of the dominant chords, as predicted by negative harmony. I find it curious that the remaining columns (hollow, avoid, stable, and modal) can be arranged to display this sine wave shape, and that Locrian, Phrygian, Aeolian, Ionian, and Lydian each have three dominants while Dorian and Mixolydian have none, though I have no idea if these observations are significant in any way:
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• ⋆ ⋆ ⋆ • ⋆ ⋆ ⋆ 1& ⋅1⋅⋅ ♭II7
⋆• ⋆ ⋆ ⋆• ⋆ ⋆ ⋆ 11 ⋅1+⋅ ♭IIΔ Loc Phr
⋆ • ⋆ ⋆ ⋆ • ⋆ ⋆ ⋆ 21 ⋅⋅+⋅ iiᶱ7 Aeo
⋆ • ⋆ ⋆ ⋆ • ⋆ ⋆ +1 ⋅⋅11 iii7 Ion Lyd
⋆ • ⋆ ⋆ ⋆ • ⋆ ⋆ +2 ⋅⋅⋅1 III7
⋆ • ⋆ ⋆ ⋆ • ⋆ ⋆ 12 ⋅⋅⋅+ ivᶱ7
⋆ ⋆ • ⋆ ⋆ ⋆ • ⋆ ⋆ 11 ⋅⋅1+ iv7 Loc Phr Aeo
⋆ ⋆ • ⋆ ⋆ ⋆ • ⋆ &1 ⋅⋅1⋅ V7 Ion
⋆ ⋆• ⋆ ⋆ ⋆• ⋆ 11 ⋅+1⋅ VΔ Lyd
⋆ ⋆ • ⋆ ⋆ ⋆ • ⋆ 12 ⋅+⋅⋅ ♭viᶱ7
⋆ ⋆ ⋆ • ⋆ ⋆ ⋆ • 1+ 11⋅⋅ ♭vii7 Loc Phr
⋆ ⋆ ⋆ • ⋆ ⋆ ⋆ • 2+ 1⋅⋅⋅ ♭VII7 Aeo
⋆ ⋆ ⋆ • ⋆ ⋆ ⋆ • 21 +⋅⋅⋅ viiᶱ7 Ion
⋆ ⋆ ⋆ • ⋆ ⋆ ⋆ • 11 +1⋅⋅ vii7 Lyd
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Key:
+ & Δ = major 7 (not tritone substitution!)
triads 0 1 Stable P1 P5 Modal m3 M3 Leading m6 M7
sevenths 1 2 Unstable M2 P4 Hollow M6 m7 Avoid m2 TT
I’m curious what has piqued your interest in VI and VII? Here are the rest of the supermodal chords, which I also find pretty interesting. I couldn’t figure out any clever arrangement of the columns here, so I just left them the same as above. Note that a couple of these (♭II, II, ♭V) are available as triads, just not as the specific seventh listed:
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• ⋆ ⋆ ⋆ • ⋆ ⋆ ⋆ ⋅+ ⋅111 ♭iiᶱ7
• ⋆ ⋆ ⋆ • ⋆ ⋆ ⋆ ⋅& ⋅1⋅1 ♭ii7
• ⋆ ⋆ ⋆ • ⋆ ⋆ ⋆ 1& ⋅1⋅⋅ ♭II7
⋆• ⋆ ⋆ ⋆• ⋆ ⋆ 1⋅ 1&⋅⋅ IIΔ
⋆ • ⋆ ⋆ ⋆ • ⋆ ⋆ ⋅⋅ 1&⋅1 ♭iiiᶱ7
⋆ • ⋆ ⋆ ⋆ • ⋆ ⋆ +2 ⋅⋅⋅1 III7
⋆• ⋆ ⋆ ⋆• ⋆ ⋆ ⋅2 ⋅⋅⋅& IIIΔ
⋆ • ⋆ ⋆ ⋆ • ⋆ ⋆ 12 ⋅⋅⋅+ ivᶱ7
⋆ ⋆ • ⋆ ⋆ ⋆ • ⋆ ⋅⋅ 12⋅+ ♭v7
⋆ ⋆ • ⋆ ⋆ ⋆ • ⋆ ⋅⋅ 12⋅+ ♭V7
⋆ ⋆ • ⋆ ⋆ ⋆ • ⋆ 12 ⋅+⋅⋅ ♭viᶱ7
⋆ ⋆ • ⋆ ⋆ ⋆ • ⋆ ⋅2 ⋅+⋅1 ♭vi7
⋆ ⋆ ⋆ • ⋆ ⋆ ⋆ • ⋅⋅ 11+1 VI7
⋆ ⋆ ⋆• ⋆ ⋆ ⋆• ⋅+ 11⋅1 VIΔ
⋆ ⋆ ⋆ • ⋆ ⋆ ⋆ • ⋅+ 11⋅1 ♭viiᶱ7
⋆ ⋆ ⋆ • ⋆ ⋆ ⋆ • ⋅1 11⋅+ VII7
⋆ ⋆ ⋆• ⋆ ⋆ ⋆• ⋅1 11⋆+ VIIΔ
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I can even provide a list of all 48 diatonic seventh chords if you like! I tend to agree though about the usefulness of negative harmony though. Other than finding questionable alternative dominants, I’m not really sure what else to do with it.