Like solving a Rubik's cube, we're working step by step to show how the whole universe of chords in rock songs can unfold from the basic three chords, [one], [four], and [five].
First we learned that there's a minor mode with its own three chords, [one]m, [four]m, and [five]m.
Next we learned about borrowed chords. The key of C major can borrow chords from its parallel minor key, which is C minor. Conversely, C minor can borrow chords from C major.
Now it's time to look at another minor key that has a close relationship with C major. This is its relative minor key, the key of A minor.
C major and A minor share the exact same set of notes, which happen to be the white keys on a keyboard. The only difference between the C major and A minor scales is in the keynote, the first note of the scale — obviously, C for C major and A for A minor.
Every major key has a relative minor key with a keynote (or tonic) a minor third lower, and the two keys share the same set of notes.
This relationship cuts both ways. Every minor key has a relative major key with a keynote a minor third higher. It's almost like two sides of the same coin. Or like Dr. Jekyll and Mr. Hyde, if you will. Bruce Wayne and Batman. David Johansen and Buster Poindexter. The same person with two different identities.
Now, let's look at the tonic chords of C major and A minor. The C chord contains the notes C, E, G. The Am chord contains A, C, E. The chords have two notes out of three in common. If you're playing a three-note C chord, raise the G one step to A, and the chord becomes Am. Lower that note to a G again, and you're back to the C chord.
In the key of C major, Am is the [six]m chord. The [six]m chord is the next most important chord in rock music after [one], [four], and [five]. It allows for this chord phrase, so popular in rock music c. 1960 that they wore it out:
[one] [six]m [four] [five]
C | Am | F | G
And this chord phrase, featured in so many 1990s hits that you might want to use it sparingly today:
[one] [five] [six]m [four]
C | G | Am | F
As a songwriter, what can you do with the special alter-ego relationship between [one] and [six]m? Quite simply, you can use [six]m in place of [one] just about anywhere! Take any three-chord phrase, and drop in [six]m wherever [one] appears. Suddenly you have a new chord phrase. It still makes musical sense, but it's more subtle, less obvious.
And just to blow this door wide open, the [four] chord also has its own alter ego, [two]m. And [five]'s alter ego is [three]m. In the key of C, we're talking about F and Dm as alter egos, and also G and Em.
Now we have two complementary trios of chords, for a total of six. There is more to this story, so come back tomorrow!