Of all the harmonic devices I use regularly in my composing, none is more useful than non-diatonic chords a third apart.
By "a third apart," I mean the interval between the roots of the two chords. The interval C to E is a major third, thus the chords C major and E major are a major third apart. C major and Eb major are a minor third apart.
In simple diatonic harmony, all chords share notes from the same key. So in diatonic C major, if your first chord is C and you go up a third, you get to E minor.
The technique we're going to talk about here is using chords a third apart that are non-diatonic. So instead of C to Em we will use C to E. The E major chord has a G# which is not part of the key of C, thus it is a non-diatonic chord.
More Major and More Minor
There is an unusual phenomenon about chords that I have written about in the past: non-diatonic major chords tend to stick out as "more major", and non-diatonic minor chords tend to stick out as "more minor".
What is meant by "more major" is that a non-diatonic major chord tends to take on more of the characteristics we associate with major. They feel brighter, happier, more positive and so on. "More minor" means the chords feel even darker, sadder, stranger than usual.
Let's first demonstrate "more major". Here I go back and forth from C major to E minor:
The Em chord feels pretty sad, as minor chords tend to sound. Let's simply change it to E major:
The E chord feels very uplifting, alive and powerful.
Now let's try a different non-diatonic major chord a third above C. Instead of a major third up we'll go a minor third up, to Eb:
It feels very bold and positive.
Now let's do the reverse to demonstrate how non-diatonic minor chords can feel "more minor".
We'll begin with Cm going a diatonic minor third up to Eb:
First we'll change that Eb to Ebm. So here is Cm to Ebm:
Kind of creepy right? Now let's try going up a major third to non-diatonic Em:
It's very dark, and even feels like a much bolder statement than the Ebm.
For one last example let's combine the ideas. We'll start out with C major and move up a third to the non-diatonic chord Ebm:
Now that's dark! All three pitches of Ebm - Eb Gb and Bb - are outside of the key of C. Plus that first C major chord grounded us in a context of C major, making the Ebm seem very foreign.
Put it To Use
Although the chord changes sound fresh and different than traditional Mozart or your average One Direction song, this is a simple idea you can start using right away.
Just go up or down a third (major or minor) to a chord with the feeling you want. (Extra credit: since the tritone is just two minor thirds away, it works the same way as well, eg. C to F#.)
If you are trying to convey royalty you might think of strength and nobility. To me that obviously suggests major chords, and so I might go C, down a major third to Ab, down a minor third to F, back home to C. All major of course:
Winter is coming and there's an ominous omen on the horizon? Sounds like minor chords to me.
For example Em, up a minor third to Gm, back to Em, down a major third to Cm:
Loops and Ambience
Movement by thirds can be a great way to keep your music moving forward without forcing traditional diatonic expectations.
For example if you simply want to convey a mood without necessarily using a theme or melody, you will probably stay pretty static on one chord.
But this can get boring after a while, so you can drive the energy forward by moving the whole bed up or down a third.
In this loop I did for a game, the music starts out in C. After about 15 seconds I take it up a minor third to Eb, then again to Gb (or F#), then again to A.
The great thing about this pattern is that by the time we loop back to the beginning we end up right back at C. So we are in continuous motion up a minor third without ever really knowing where it begins or ends.
This is Everywhere
Once you start working with chords a third apart you will begin to hear it everywhere, particularly in film and game music.
Howard Shore uses this technique constantly in his Lord of the Rings scores.
In this excerpt from "The Prophecy" starting at 2:13 almost every single chord change is either a third or tritone apart:
Or consider the famous Rebel Fanfare from Star Wars, which is parallel major triads. It starts on Bb and goes first down a minor third to G, then up a minor third to Db. At 1:55 in this excerpt:
The important thing here is to understand the difference between diatonic and non-diatonic chords, because only then can you take advantage of the unique aspects of the non-diatonic chords.
Whether to go up or down, and by a major or minor third, is a question of context and taste. I have found no formula that says in some situations "going up a minor third" is better than any other choice. You will have to use your ears!
For a much deeper look into the complex theory behind why this technique sounds good, I highly recommend Richard Cohn's "Audacious Euphony". (Music theory nerds only!)
I hope this technique expands your harmonic vocabulary both as a writer and a listener. Share your favorite examples or your own creations in the comments.