by Gary Ewer, from “The Essential Secrets of Songwriting” website:
One way to inject some interest into your chord progressions is to experiment with substituting one chord for another. But rather than making it a mindless activity, consider chord categories. The three basic categories that we will consider for this article are thetonic, dominant, and predominant chords.
Within a chord progression, each chord has a particular function. For example, the I-chord (the chord representing the key) is called a tonic chord, and it has, not surprisingly, a tonic function. Chords that move easily to the I-chord are said to have a dominant function. The chord that best represents this function is the V-chord. And chords that move easily to dominant chords are called predominant chords.
For each of these categories, there is one chord that is the most obvious and strongest example. For tonic chords, as mentioned, the I-chord (being the tonic chord) is the best representative of this category. But there are other possibilities. The vi-chord makes a good substitute for the I-chord. Consider this progression from the key of C major:
C F G C
We can represent each of these chords with Roman numerals: I IV V I
Now, instead of ending this progression with a I-chord (C), we can end it with a vi (Am):
C F G Am
The Am has a tonic function, because it serves as a good ending to that progression, and that’s what a tonic chord does: It serves as “home.” So vi-chords can have a tonic function, though weakly; we’d probably want to end our song eventually on a I-chord, because I-chords have a stronger tonic function than vi-chords.
With predominant chords, we see that chord as wanting usually to move to a dominant chord. The most common predominant chord is IV, but we can replace IV-chords with other chords that have a different quality. For example, with the progression mentioned earlier (C-F-G-C), we can substitute the IV-chord (F) with a ii-chord (Dm). Since F and Dm have two notes in common, they often are quite interchangeable.
With dominant chords, the most common one is the V-chord (G, in our example). But you’ll find that Em (iii) has a dominant quality, and can serve as a dominant chord.
So to simplify this, here are a list of chords, and some possible replacements to consider. (All examples are from the key of C major):
Best: C (I); Other possibilities: Am (vi), F (IV)
Best: G (V); Other possibilities: Em (iii), Bdim (vii)
Best: F (IV); Other possibilies: Dm (ii), Am (vi)
So with our example progression (C-F-G-C) we could make these possible substitutions:
C Dm G C
C Dm Em C
C F G Am
C F Em Am