There are many improvisers out there who can hear a tune and instantly create a rich palette of chords to support it. If you asked them how they did it, you might receive a response like, “you just hear it in your head and you kinda know what’ll work”. Unfortunately, that’s how it’s done, and more unfortunately, that response doesn’t help you achieve much.
So here are a couple of tips to help you add chords to a melody.
Know the qualities of each type of chord
Before I play a maj7 chord, I already know that it will produce a floating, unresolved effect. Other words I might use to describe the chord are “airy, light, rich, jazzy”. In comparison a diminished chord is dark, sinister, and sounds like a sadder version of a minor chord (I loosely call dim chords double minor because it contains two minor 3rds stacked on top of each other). A sus chord has strong major sound but sounds like it’s going somewhere – it needs to be resolved.
In my head, I’ve got a worded description tagged to each type of chord, so if I hear or create a melody, and I want to produce a certain effect at some point in the melody, I know roughly which type of chord(s) to choose from. Often we sit at the piano and say things like, “For this spot in the melody, I can sorta hear the chord that goes with it” and we proceed to randomly mash a few chords but it’s not quite what we heard. If that happens a lot, take heart – at least you’re hearing something, which means the difference between you and a professional is that they know which chord they’re hearing and you don’t. Once you work it out, you’ll remember it and the next time you hear the same chord in your head, you’ll know what it is.
If however you don’t hear anything in your head, then you can either randomly play some chords and hope for the best, or use some theory. I recommend the latter.
Use a bit of theory
Let’s say your song is in the key of C, and you’re trying to find a chord that will work with the current melody note B.
Jot down all the chords that you know that contain a B and begin with the obvious ones. G and Em are the only triads in the key of C that contain a B, so try them first. If they don’t produce the sound you’re after, try some 4 or 5 note voicings that contain B but stick within the key of C (ie. white notes only), eg. Cmaj7, Em7, G7, Am9, Fmaj7#11, etc. If that still doesn’t work, then try chords from different keys that contain B, like E, F#sus, B, etc. There are also numerous jazz voicings that may work like upper structures.. try playing an E root position triad in the RH while playing F and B in the left (your chord should read F B E G# B from left to right). Now there’s a cool way to accompany your B melody note! OK, now I digress.
Although this provides a system for you to find the right chord, our goal is not to go through 50 chords each time using a wonderful system. The aim is obviously to get to the desired chord with as few attempts as possible. And this brings us back to the hearing side of things. The more chords you experiment with, the better you’ll know what they sound like. For beginners, start by asking a friend to play a bunch of triads in various inversions and you try to distinguish between major and minor triads. Then work in a few sus chords and slash chords like C/E, G/F, F/G. Then you’re ready to train your ear at 4 note voicings like Am7, Cmaj7 and so on.