We can create some more interesting and unusual chord progressions by using major chords which are derived from the major key's parallel minor scale. This is simply the minor scale that shares the same root note as the major key you are playing in. So, if you were playing a song in the key of G major, the parallel minor scale would be G minor.
In practice, this means we can insert a IIIb major, VIb major and VIIb major chord into our progression (when compared to the major scale), since these are the chords that have a major third in the parallel (natural) minor scale.
We call this process modal interchange. Let's explain in more detail. If we take the C major scale:
The III b major of this scale is Eb. The VI b major is Ab, and the VII b major is Bb.
We can then use these three chords in addition to our normal chords in the key of C. Using these chords give the music a hard edge, and are commonly used in hard rock songs.
We can apply this to any key. For example, if we were creating a chord progression in the key of A major, we could then use C major (IIIb major), F major (VIb major) and G major (VIIb major) in addition to the existing diatonic chords.
Perhaps the most famous use of modal interchange is the song 'Hey Joe' by Jimi Hendrix, in the key of E major:
ListenHere we see the use of the flat third, sixth and seventh major chords in the key of E (E major would normally contain the notes C#, G# and D# as the VI, III, and VII chords). We can deduce that these chords come from the E (natural) minor scale, and can be used interchangeably in both major and minor keys.
So far we have looked at using major chords taken from the parallel minor scale. What about the minors?
A common occurence of modal interchange using minor chords is to borrow the IV minor chord from the parallel minor key, which in effect is just using the IV chord of the major key and making it minor. It has a very good effect when preceeded by the IV major chord. This minor chord works because it contains a VIb tone in relation to the key.
All other minor chords from the minor key can also be used, but in practice they are rare since they sound relatively weak and uncertain.
