From “The Essential Secrets of Songwriting” website:
In the previous article, I discussed strong and fragile chord progressions, and the need to have both in your songs. But how do you actually choose the chords? How do you know which chords go with the key of your song? And how to you get them in the right order? In this article, I’m going to show you how to narrow down the thousands of possible chords to just the ones you’ll likely want to use.
In the next article (to be posted later today) I’ll go step by step, with sound samples, to show you how to harmonize a sample melody. But first… which chords go with the key you’re in?
If the song you’ve written is in D major, it means that most, if not all, of the pitches in your melody come from the D major scale: D E F# G A B C#. You can build chords on top of each note of the scale. Assuming you make no extra modifications, a chord built on the first note of any major scale will be major, a chord built on the 2nd note will be minor, on the 3rd: minor; the 4th: major; the 5th: major; the 6th: minor; and the 7th: diminished.
So here, then, are the chords that typically work with D major:
D Em F#m G A Bm C#dim
You’ll find that certain chords are used more than others. For example D, G and A are probably the most commonly used. Together, those three chords create very strong progressions.
That process we used to find the chords that fit D major will work with any key. So no matter what key you’ve chosen for your song, you’ll be able to get the chord set that belongs to that key.
Come back later for the next article… we’ll take an actual melody and harmonize it step-by-step.