I’ve been playing around with circle diagrams. We are titling the following chart “Diatonic Chord Orbits”:

It is a nice way to demonstrate the relationships and shared tones between dominant and leading tone sevenths with other diatonic chords.

Note that the inner circle contains only primary chords and that all the major scale tones are covered. This means that you can harmonize any note in the major scale with just primary chords. The second orbit contains secondary chords and the half-diminished leading tone seventh.

The only chord that doesn’t fit in either orbit is the ii chord. Why does that chord not quite fit? Below is an example of chord orbits using a ii, V, and I chords. Like primary chords, all notes of the major scale can be harmonized with one of the three chords:

Some questions still need follow-up:

1. What is unique about the ii chord that makes it not fit the diagram?
2. What is the significance of the gap in the ii-V-I diagram?