Watch the Week 2: Chord Substitutions online guitar lesson by Rob Garland from Songwriting For The Guitar
You can substitute the iii chord for the I chord in a I-vi-ii-V chord progression. This works because the chords share two of the same notes. Here we'll look at experimenting with changing the order and substituting the I for the vi chord which also share two notes and are the relative major to minor chords of the key. Sing the same vocal melody over chord substitutions to increase interest in a section of a song.
NOTE: There are many more substitutions, so experiment!