Harmonising a melody – free music theory course

Harmonising means finding chords to support a given melody. It's the most useful exercise there is for a pianist: it's what you do to accompany, arrange and improvise. Here is a six-step method, applied to “Twinkle, Twinkle, Little Star” (the traditional French tune “Ah ! vous dirai-je, maman”).

1. Key and cadences

Identify the key (here C major) and the phrase endings. The melody ends on C preceded by D: that's where a perfect cadence V → I goes.

2. Harmonic rhythm

Decide how many chords per bar. Here, one chord per half bar works well: enough motion without clutter.

3. Chord tones on strong beats

For each segment, list the chords containing the important melody note (usually the one on the strong beat). C can be harmonised by C, F or Am; A by F, Dm or Am; G by C, Em or G… Several choices are possible: this is where the art begins.

4. The bass

Choose the chords with the bass line in mind: it should be beautiful in itself, preferably moving in contrary motion to the melody. Here: C – E | F – E | D – C | G – C, an almost entirely stepwise bass, with a fifth leap for the cadence.

5. Inner voices

Fill in the missing notes following the voice-leading rules: keep common tones, move short distances, no parallel fifths or octaves.

6. Check by ear

M:4/4
L:1/4
%%staves {RH LH}
V:RH clef=treble
V:LH clef=bass
K:C
[V:RH] "C"[Gc] [Gc] "C/E"[cg] [cg] | "F"[ca] [ca] "C/E"[cg]2 | "Dm"[Af] [Af] "C"[ce] [ce] | "G"[Bd] [Bd] "C"[Gc]2 |]
[V:LH] C,2 E,2 | F,2 E,2 | D,2 C,2 | G,,2 C,2 |]

Notice the choices: the first-inversion C/E avoids repeating the bass C and creates a rising line; Dm (ii) replaces F (IV) in bar 3 to approach the dominant with a stepwise bass; the final cadence is a perfect cadence.

Start simple: with only I, IV and V (C, F, G) you can harmonise a huge number of traditional songs. Then add vi and ii for colour, and inversions for the bass line.

Common mistakes: changing chord on every note (too heavy); harmonising a passing note as if it were structural; ending on anything other than I; using V → IV in a classical style.