Borrow Chords from Related Keys
Finding a chord progression’s parallel major or minor key is easy: simply take the letter name of the original key and then flip it to major or minor. For example, the parallel minor of a progression in A is A minor, and the parallel major of A minor is A. In the realm of music theory, it doesn’t get much easier than that!
Example 5 cycles through a progression in the key of A minor, but borrows the V chord from the parallel major key of A major. The major V (E) resolves much more strongly to the Am chord than the minor v (Em), and you can hear the difference if you substitute an Em for the E major chord in this progression and compare how the two resolve to Am. Artists like the Beatles sometimes took this a step further by borrowing two chords from the parallel major. Example 6 switches the Am at the end to an A major chord, similar to how the Beatles played “I’ll Be Back.” Swapping between major I and minor i chords is less common than using the major V in a minor context, but it’s another cool trick to have in your bag.
Excerpted from Acoustic Rock Essentials