by Gary Ewer, from “The Essential Secrets of Songwriting” website:
A non-diationic chord is one which does not normally belong to your key of choice. In the key of A major, the seven naturally-occuring chords are: A, Bm, C#m, D, E, F#m, and G#dim. Of those chords, there are countless ways of modifying them by adding tones (Aadd9, for example), using non-chord-tones (Esus4, for example), and so on. Of those seven chords, the ones you would find yourself using the most are probably I, IV and V (A, D and E in our example of A major)
Then there is a different category of chords called non-diatonic chords. These are ones which cannot be formed by building simple triads on the notes of the scale. Some of these chords have categories all their own. For example, secondary dominant chords are formed when you take a chord that is normally minor in a certain key and making it major, then following it with a chord four notes higher. Here’s an example:
A F#m B E A
The B chord is the secondary dominant. You would expect that chord to be Bm in our key of A major. By making it a major chord, then following it with a chord four notes higher (E), you’ve created a secondary dominant.
I want to focus, however, on the non-diatonic chords that don’t have roots that exist in the key. For example, the flat-3 chord. In A-major, the flat-3 chord is C. It can have a somewhat startling sound:
A E F#m A C D A
The great thing about the flat-3 is that it adds a certain moody colour to the sound of your progression. But here’s a way that it can have even better effect: Use it sparingly.
Keep in mind that chord progressions represent a type of groove that your song needs. For many songwriters creating chord progressions, once they’ve created one chord, they look for a new one, then another, then another, and eventually their progressions sound like a bit of a mess! The more chords you add to your progression, the greater the danger of killing the sense of groove in your songs.
So especially if you plan to use a non-diatonic chord like a flat-3, set up a groove using two basic chords, maybe a third chord for variety, from your chosen key, go back and forth between those chords to establish the groove, and then throw in the flat-3. You’ll love the result:
A Bm A Bm A Bm A D A Bm A Bm C D A
That C chord jumps out and demands attention when it happens, but it does so in a really great way: Every groove needs a bit of shaking up just for variety, and that’s the power of the non-diatonic chord. So just throwing non-diatonic chords everywere may be a little like making a soup where you throw in every spice in your rack: none of them are being used to good effect.
So set up a chord groove, and then let that non-diatonic chord pleasantly disrupt proceedings!
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