Choosing chords under a melody

You've written a melody, it sings on its own… and now you'd like to hear it dressed up. Harmonising means choosing, for each moment of the melody, the chord that supports it. Good news: with just three chords — degrees I, IV and V — you can harmonise the vast majority of simple major-key melodies. This lesson gives you a step-by-step method, the one composers have used from Bach to modern songwriters.

1. The three pillars: I, IV and V

In C major, the three primary chords are C (C–E–G, degree I), F (F–A–C, degree IV) and G (G–B–D, degree V, often enriched as G7 with an F). Between them they contain every note of the scale. Look at this table: for each melody note, it tells you which chords contain it.

This table is your compass. When a note belongs to two chords (C, G, F), that's where you have a real composer's choice — and that's where colour is born.

2. The golden rule: chord tones on strong beats

You don't harmonise every note. First look at the strong beats (beats 1 and 3 in 4/4): they are the ones that "carry" the harmony. Notes on weak beats may lie outside the chord, as long as they move by step: these are passing notes or neighbour notes. Here is a four-bar melody harmonised with one chord per bar.

M:4/4
L:1/4
Q:1/4=88
K:C
%%staves {RH LH}
V:RH clef=treble
V:LH clef=bass
[V:RH] "C"E2 D C | "F"A2 c A | "G"d2 B G | "C"c4 |]
[V:LH] [C,E,G,]4 | [F,A,C]4 | [G,B,D]4 | [C,E,G,]4 |]

Bar 1: E (beat 1) and C (beat 3) belong to C major; the D on beat 2 is a passing note. Bar 2: A and C belong to F. Bar 3: D and G are in G. Bar 4: back to C, the phrase is closed.

3. Non-chord tones: don't panic

A flowing melody inevitably contains notes that aren't in the chord. As long as they sit on a weak beat and link two chord tones by step, the ear accepts them and even enjoys them: they are what creates motion.

M:4/4
L:1/8
Q:1/4=80
K:C
%%staves {RH LH}
V:RH clef=treble
V:LH clef=bass
[V:RH] "C"c2 BA G2 E2 | "F"F2 GA c2 A2 | "G"B2 cd B2 G2 | "C"c6 z2 |]
[V:LH] [C,E,G,]8 | [F,A,C]8 | [G,B,D]8 | [C,E,G,]8 |]

Bar 1: B and A link C to G — passing notes. Bar 2: G links F to A. Bar 3: C links B to D. You see the principle: a non-chord tone is "passing through", never "sitting down".

4. One note, several colours

The note E belongs to C major (its third), A minor (its fifth) and E minor (its root). Listen to how the same melody changes emotion depending on the chord: bright over C, wistful over A minor, floating over E minor.

M:4/4
L:1/4
Q:1/4=72
K:C
%%staves {RH LH}
V:RH clef=treble
V:LH clef=bass
[V:RH] "C"E4 | "Am"E4 | "Em"E4 | "C"E4 |]
[V:LH] [C,E,G,]4 | [A,,C,E,]4 | [E,G,B,]4 | [C,E,G,]4 |]

Remember this: harmonising isn't finding the right answer, it's choosing a light. The secondary chords (ii = D minor, iii = E minor, vi = A minor) often stand in for I, IV or V for extra nuance: vi replaces I, ii replaces IV.

5. The five-step method

1. Identify the key and the final note (usually the tonic).
2. Circle the strong-beat notes in each bar.
3. List the possible chords for each one using the table.
4. Start from the end: the last bar is almost always I, the one before almost always V. Fix those two chords first.
5. Fill in the rest, favouring variety and natural motion (I → IV → V → I is the backbone).

Your workshop

1. Open the piano roll and write (or reuse) an 8-bar melody in C major, in half notes and quarter notes.
2. On paper, note the beat-1 and beat-3 notes of each bar, then the possible chords (I, IV or V) from the table.
3. Fix the ending first: bar 7 = G (V), bar 8 = C (I). Then complete bars 1 to 6.
4. Write the accompaniment as block chords (whole notes) in the left hand, between C2 and C4. Listen.
5. Replace one C chord with A minor (vi) and one F chord with D minor (ii): compare both versions and keep the one you prefer.
6. Self-check: on every strong beat, is the melody note part of the chord? Does your phrase end with V → I?