About the ♭VII chord: How is that ranking derived? The Trends API always converts Theorytabs to the relative Major key before evaluating the chord progressions, and it does not keep track of the original modes of the Theorytabs. So in practice all these progressions refer to the same I -♭VII progression in the popular music style, but only when the respective Theorytabs’ modes are also taken into consideration:
- Lydian “4.D3”
- Major “1.M7”
- Mixolydian “5.4”
- Dorian “L2.1”
- Minor “5/2.5”
- Phrygian “5/6.L2”
- Locrian “5/3.5/2” etc.
And these are identical to i - ♭VII:
- Lydian “b4.D3”
- Major “D1.M7”
- Mixolydian “M5.4”
- Dorian “2.1”
- Minor “6.5”
- Phrygian “3.L2”
- Locrian “L7.5/2” etc.
Furthermore, each chord in the API is equivalent to a number of other borrowed chords of the relative Major mode, so “M1.b7”, “4/3.L5”, “b5.D4” and so on refer to the same I(i) -♭VII progression. I believe this constructed ranking reveals major flaws in the unfinished Trends API, which does not even encompass all usable chords, nor accurately reflect the ubiquity of ♭VII (in fact I tend to skip analyses of parts that only use “6.5.4.5/6” and the like).
To derive a ranking like that, for instance, the Trends API must search for “6.5” but remove results from Theorytabs that are not in the Minor mode, so that progressions like the Major “1.4.6.5” won’t be misinterpreted as i -♭VII, and so on for all other non-Minor progressions using “6.5”, and so on for equivalent chord progressions in other modes.
Indeed, it is counter-intuitive to refer to chords as borrowed chords in the relative major in the Trends API, when “1” does not always mean the tonic. From a statistical viewpoint, one does not even need to know that the Major ii - V is identical to the Dorian i - IV and the Mixolydian v - I, as chord progressions are treated with respect to the tonic in each mode. The Trends API should always convert Theorytabs to the parallel Major key rather than the relative, and the common practice tenet of shifting the diatonic scale to reach other modes ought to be dropped in the Trends API. The Trends API should make it clear that the Major “6.4.1.5” (by repeating “1.5.6.4”) is different from the Minor “6.4.1.5”, which is equivalent to Major “b1.b6.b3.b7” as borrowed chords have worked in the Major mode as a compatibility workaround. The same applies in general when non-diatonic modes such as Harmonic Minor and Freygish are added to Hookpad.
Then, using the popular music style, the Trends API could unite chord progressions from all modes, even compatibility Theorytabs that use the parallel major key of a minor key, and other mixed-mode progressions like IV - V - ♭VI - ♭VII - I. The chord quality of a common practice Roman numeral can be determined from its scale degree and mode, but for the popular music style, both the required accidentals and the chord type must be explicitly shown in the API. Now that would be a much larger update compared to simply changing the display of Roman numerals in non-Major modes.