Rule 2. Each harmonic triad defines this pair.
E.g, the 3M7-6m-2m harmonic triad defines the 3-2 isokratics (or ancient power chords 3P-2P, where the 6m is usually substituted by 3P, and rarely by 2P) , or the even-odds chord arpeggios. 3-5#-7-2=3M7 and 2-4-6-1 =2m7 .
Rule 1 This rule is supplemented with the rule where any triad of any mode of a diatonic scale is substituted wth the triad of a single mode through substitutions by relative chords. E.g. If in a song there are the chords 5M, 1M, 4M, but also 2m, 6m, 3m, and 7d then to absorb all chords only in the triad 5P, 1P, 4P, the 2m is substituted with 4M (relative chords) thus eventually 4P , 6m by 1M (relative chords), thus by 1P, and 3m and 7d by 1M7 (again, relative chords) thus by 5P.
With these two rules, we can convert an ordinary chord progression of minor and major chords to only 3 or 2 ancient power chords.
But conversely, it can be used to discover an ordinary chord progression of minor and major chords of a melody by starting at first with an harmonization with only 2 or only 3 ancient power chords.
https://www.youtube.com/watch?v=kuCGvw-N_BA
The idea, of course, behind the pair of isokratic notes differing by an interval of a 2nd in a mode of a scale is that any melody on a mode of a scale can be harmonized with 2 chords: the odd notes chord corresponding to the note number 5 and the even notes chord corresponding to the note 4.
When in a song, which is in a scale and mode, of notes, numbered 1,2,3,4,5,6,7, the isocratic notes are 4-5, then, the bass line, is the notes 2-4-6-1 as long as isocratic note 4 lasts and 1-3-5-7 as long as isocratic note 5 lasts. The isokratic notes are also ancient power chords (neither major neithe minor, made from interval of 8th and 5th like C-C'-G')
This bass method, in a 1,2,3,4,5,6,7 mode-scale, is based on the musical theorem, in harmony, that every melody in such a mode can be harmonized with two single 4-chord chords, 1-3-5-7 and 2-4-6-1. So the bass lines are arpeggios on these two four-chord chords. We always talk about diatonic modes or modes in melodic or harmonic minor.
