Harmonising a tune you know · From Zero to Mozart

Three chords — I, IV and V — can accompany a startling amount of music. Here's why, and how to work out which one goes where.

Why three chords is often enough

I contains ^1 ^3 ^5 (do–mi–sol)
IV contains ^4 ^6 ^1 (fa–la–do)
V contains ^5 ^7 ^2 (sol–ti–re)

Between them, those three chords hold all seven degrees of the scale. So whatever note the melody is on, at least one of I, IV, V has it — and usually the ear tells you which.

The rough rule:

a bar leaning on do / mi / sol → I
a bar leaning on sol / ti / re (or fa) → V (or V7)
a bar leaning on fa / la → IV

Ode to Joy, harmonised

The tune sits almost entirely on I and V, with one IV near the end. Play the melody, and where the chord symbol changes, hear the harmony underneath shift:

90 bpm

Ode to Joy, first phrase, in G major. Bars 1–3 rest on notes from the tonic chord (B, C, D, G, A are mostly ^3 ^4 ^5 ^1 ^2) — so: I. The last bar lands on A (^2), part of the V chord, and it sounds unfinished — a half cadence.

Try it

Try

Turn on the drone bar with a G drone and play the Ode to Joy phrase above. For the first three bars the melody agrees with the drone — it's all tonic. On the last bar, the A grinds a little against the G: that's the melody wanting a V chord instead of the tonic drone. You can hear where the harmony needs to change.

Which chord?

The usual mistake

Forcing a chord change every bar, or never changing at all. Let the melody tell you: when it moves to notes that clash with the current chord and agree with a new one, that's your cue. And a tune can happily sit on one chord for several bars — Twinkle's whole first line is just I, then IV, then I.