Watch the Sub a Third Above or Below online guitar lesson by Fareed Haque from Solo Guitar Handbook
Principle #1: Sub a Third Above or Below
The basic theory here is really simple: Since chords are built in thirds, chords a third apart share lots of common tones.. IN our tune Amazing Grace, we can use a C major 9 chord. Just stack thirds in the key of C up to the 9th: 1-3-5-7-9 = C-E-G-B-D. So lets compare the C maj9 to the chord built a third higher, built on E. Now it's simple to see that in that C major 9th chord there is a little Em7th chord (C-E-G-B-D). So, try subbing E minor for Cmaj. Now lets go a third BELOW C maj9, or 'A'. notice that Am9 is spelled A-C-E-G-B. Do you see the little C major 7 hidden inside A minor 9? A-C-E-G-B. So, try Am instead of C major and see if that's nice.
Hey, if this confuses you, to understand music theory there's really only a few things to know: Know your keys and know how to spell chords. If you can spell 'em, the rest follows very easily. It's just like knowing multiplication tables, or the alphabet: If you don't know that, then math or reading will forever be confusing...so just learn it and get 'er done. Check out Appendix A if you want some help drilling your chords and keys.