As it is known in a diatonic scale there are 3 triads of chords
1) The triad of majors 1M-4M-5M
2) The triad of minors 3m-6m-2m
3) The minors-diminished triad 7d-3m-6m
By converting the latter 2 triads to majors we have the
2b) 3M-6M-2M which is also the triad of majors of 6 major
3b) 7M-3M-6M which is also the triad of majors of 3 major
By doing so we get also all the 5 chromatic notes relative to the original 1 major diatonic scale in other words the 1# , 2# 4#, 5# except the 6# . But the 6#=7b can also be obtained by the 1M7 thus an full chromatic tonality.
The final sequence of major chords set in the wheel by 4ths (or 5ths) is
7M7->3M7->6M7->2M7->5M7->1M7->4M7
THUS THE VERY WELL KNOWN CYCLE OF 7 CHORDS IN THE WHEEL BY 4THS!
A gradual covering of all these 7 chords can be so that at each of the 3 stages we always have one triad of majors and 2 triads of minors or diminsihed!
For example
1st phase [from 1 major scale]
(1M -4M-5M), (3m-6m-2m), (7d-3m-6m)
2nd phase [from 6 major scale)
(1m -4m-5m), (3m-6m-2m) (7M7-3M-6M)
3rd phase [from 3 major scale]
(1m -4m-5m), (3M7-6M7-2M7), (7d-3m-6m)
Finally if we want to correspond simple patterns of harmony with simple patterns of melody then
1) Threechords in other words 3 consecutive notes subscales correspond to triads of chords that have distance of roots an interval of 2nd. And there are so many such 3-scales as are the notes of each chord. Furthermore threechords occur externally as linking bridges to consecutive chords of the chord progression, when the chord progression have roots that have a distance an interval of 3rd.
2) 5-chords in other words 5 consecutive notes subscales with total length an interval of 5th occur internally to each chord of the chord progression
3) tetra-chords in other words 4 consecutive notes subscales with total length an interval of 4th occur externally as linking bridges to consecutive chords of the chord progression, when the chord progression is an arc of the wheel by 4ths or more generally the roots of the consecutive chords differ by an interval at most of 4th. This explains why when improvising over a diatonic scale and a chord progression by chords of it, each time the chord changes, it is at most within a tetrachord that contains the last "staying" note (melodic center) of the last chord that we find the next "staying" note (melodic center) of the next chord.
