And...
The Circle of 5th's and the ii-V-I are also connected this way.
Try this...start anywhere in the Circle of 5th's and make that note the Root of a m7 chord, then from there step one note backwards in the Cycle and make that the Root of a Dom7 chord, then from there step one more note backwards in the Cycle and make that note the Root of a maj7 chord...
You've just created a ii7-V7-Imaj7. The key it would be in is the Note Name of the Root of Imaj7 chord.
So, for instance in the Cycle...
Start with D, make it Dm7...now go counter clockwise one note to G, make that a G7, now go counter clockwise one more note to C, make that a Cmaj7. There you go: Dm7-G7-Cmaj7 the ii-V7-Imaj7 in the Key of C.
Another example:
Start with Bb, make it Bbm7...now go counter clockwise one note to Eb, make that a Eb7, now go counter clockwise one more note to Ab, make that a Abmaj7. There you go: Bbm7-Eb7-Abmaj7 the ii-V7-Imaj7 in the Key of Ab.
You can look at this way...
Say you know the Key but need to find the ii and the V. Find the Note Name of you Key in the Cycle, then the note clockwise from it is the V and one more note clockwise is the ii.
Both of these work everytime, flawlessly.
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