Reharmonising a Song You Already Know

First, the one piece of ground the whole exercise stands on: a chord fits a melody note when that note lives inside the chord, or brushes past it on the way to one that does. Hold a long E in the tune and C major will carry it, because E is the third of C. But E is also the fifth of A minor, and the major seventh of F. Each of those chords holds the same note in a different light. The melody does not change; the meaning underneath it does. That is the entire practice, and everything past it is taste.

Swap one chord at a time

Do not rewrite the progression. Change one chord, play the phrase, and listen to what happened to the feeling. The relative minor swap is the gentlest door in: wherever the tonic sits under a melody note it shares with its relative minor, try the minor (A minor for C, E minor for G). The tune suddenly remembers something sadder. Then try ii where IV was (D minor for F, in the key of C): the same neighbourhood, with more forward lean. Then the one worth the detour: a borrowed chord where V used to be, which I wrote about at more length in the borrowed chords primer. Each swap is a small hypothesis. The melody accepts it or it does not, and your ear files the verdict before you have named the chord. I hum it first and label it second; the order matters.

Keep an eye on the bass while you do this, because the bass is where a substitution either settles or falls over. If the roots of your new sequence move by step, or by a comfortable fourth or fifth, the change will feel intended; if the bass suddenly lurches, the ear hears the seams. Quite often the difference between a clever swap and a usable one is nothing more than which inversion you choose, and that is a matter of one finger.

The cryptic clue standard

How do you know a substitution is good rather than merely clever? Femi and I have been arguing one particular substitution since 1998 (the argument is most of the friendship at this point), and the only standard we have ever agreed on is this: a good reharmonisation lands the way a good cryptic clue does. Surprising at first glance, then, a second later, plainly fair. The listener should feel the ground shift, and then feel that the ground had every right to shift. If it only surprises, it is showing off. If it only behaves, why did you bother. The clue has to click.

A good substitution is surprising at first glance, then, a second later, plainly fair.

A practical way in

Take a song you already play, ideally one you are slightly bored of (boredom is useful here; it lowers the stakes). Lay its progression out in the chord builder, change exactly one chord, and play the melody over the top. Keep whatever makes you tilt your head. Discard whatever makes you wince. One chord per evening is plenty: this is proving, not kneading, and it rewards patience far more than force.

Yusuf arrived at his lesson this month with a tune he had reharmonised entirely on his own, most of the way to somewhere good without asking me a single question en route. I said very little, which is the highest compliment available to a teacher. Reuben used to tell me that theory is a map somebody drew after the journey. A reharmonisation is you taking the same journey by a different street, and finding your own front door still standing at the end of it.

The tune stays the tune. Everything underneath it is negotiable. Play it and listen.