To see how the Circle of Fifths can help us with the concept of borrowing chords we will look at parallel major and minor keys (not relative major and minor).
Note - borrowing chords is also referred to as modal interchange. Under that name, a more expansive concept can be explored. Here, for now, we will be limited to just a small slice of what can be done when chords are ‘borrowed’ from elsewhere.
Parallel major and minor keys are simply one major key and one minor key both of which have the same root note. They do not have the same tonic chord of course, because the tonic of one will be a major chord and the other a minor chord. Hopefully that much is obvious.
Using the three major key examples that have been consistent throughout this study we would have as parallels:
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C major and C minor
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Ab major and Ab minor
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E major and E minor
One major and one minor built from the same root note.
Major in this sense is always THE major scale (Ionian) and minor is always THE minor scale (natural minor or Aeolian) built from the 6th scale degree of its relative major.
So, C major is just the key of C major. Enough said. When we consider its parallel minor key, C minor, we must be aware that it is not the relative minor of C major. Their commonality is the root note. C minor is the relative minor of a different major key – specifically Eb major because C is the 6th scale degree of the Eb major scale.
Look on the outer and inner wheel of the Circle of Fifths above - you will see Eb and Cm in matching positions to confirm this.
That said, there is no need to think that we will be doing all sorts of mathematical equations and counting and trickery to identify what and where the parallel major and minors are. Because we have the Circle of Fifths.
The Circle of Fifths gives us an instant, visual means of finding the parallel keys and their chords. In an instant. No mathematics or tricks involved. We will see that next.