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How do you harmonise a simple melody using the primary chords and plan cadences at the phrase ends?

Harmonise a simple diatonic melody using the primary chords I, IV and V, choosing a chord for each melody note and planning perfect, imperfect and plagal cadences at phrase ends

A focused answer to the O-Level Music composing outcome on harmonisation. Choosing primary chords I, IV and V for melody notes, planning cadences at phrase ends, keeping a sensible harmonic rhythm and a smooth bass, with a step-by-step harmonisation walkthrough.

Generated by Claude Opus 4.810 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
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  3. Examples in context
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What this dot point is asking

SEAB wants you to harmonise a simple diatonic melody using the primary chords (I, IV and V): to choose a chord for each melody note and to plan cadences at the phrase ends. The central insight is that harmonisation is done from the cadences inward: you fix the cadence at each phrase end first, then fill in the body with primary chords that contain the melody notes, in a steady harmonic rhythm.

The answer

The primary chords

The three primary chords are built on the first, fourth and fifth degrees of the scale, and between them they contain every note of the major scale:

  • Chord I (tonic): scale degrees 1, 3, 5.
  • Chord IV (subdominant): degrees 4, 6, 1.
  • Chord V (dominant): degrees 5, 7, 2.

So any melody note can be harmonised by at least one primary chord (for example the note that is degree 2 belongs to V, degree 4 belongs to IV).

Plan the cadences first

The phrase ends are the structural pillars, so decide their cadences before anything else, choosing the chords from the melody's ending notes:

  • Perfect cadence (V to I): conclusive, for a phrase ending on the tonic.
  • Imperfect cadence (ending on V): open and unfinished, for a mid-point pause.
  • Plagal cadence (IV to I): the gentle amen, an alternative close on the tonic.

Choose a chord for each note

Working between the cadences, give each main melody note a primary chord that contains it. Where a note fits more than one chord, choose the one that flows best from the previous chord (smooth progressions include I to IV, IV to V, and V to I). Keep a steady harmonic rhythm, often one chord per bar or per two beats, rather than changing chord on every note.

Keep the bass smooth

Put the chord roots in the bass, but choose octaves and the chord order so the bass line moves reasonably smoothly rather than leaping wildly. A bass that mostly steps or makes small leaps supports the harmony better than one that jumps about.

Examples in context

Example 1. A hymn harmonised with primary chords. Many simple hymns are harmonised largely with I, IV and V, a perfect cadence ending each verse and the occasional plagal amen. They show how far the three primary chords alone can go in supporting a singable melody.

Example 2. A folk song with a guitar accompaniment. A folk song accompanied by three guitar chords is, in effect, harmonised with the primary chords of its key, the player changing chord in a steady rhythm and ending on the home chord. It is everyday proof that I, IV and V can harmonise a whole tune.

Try this

Q1. Name the primary chords and the scale degrees each contains. [3 marks]

  • Cue. Chord I contains degrees 1, 3, 5; chord IV contains 4, 6, 1; chord V contains 5, 7, 2.

Q2. A phrase ends on the tonic. Suggest a cadence and the chords, and name it. [2 marks]

  • Cue. Approach the tonic with V to I for a perfect cadence (conclusive), or use IV to I for a plagal cadence.

Q3. Explain why you should plan the cadences before harmonising the rest of the melody. [2 marks]

  • Cue. The phrase-end cadences are the structural pillars; fixing them first gives the harmonic goals around which the chords of the body are chosen.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original8 marksDescribe how you would harmonise a simple eight-bar melody in C major using only the primary chords, including how you choose a chord for each note and how you plan the cadences at the two phrase ends.
Show worked answer →

Identify the primary chords. In C major they are I (C, E, G), IV (F, A, C) and V (G, B, D). Each scale note belongs to one or more of these chords: for example E is in I, F is in IV, B is in V.

Plan the cadences first. Decide the cadence at each phrase end from the melody note. If the first phrase ends on the supertonic or leading note, harmonise it with V for an imperfect cadence (open). If the second phrase ends on the tonic, harmonise it V to I for a perfect cadence (closed); a melody ending that suits IV to I gives a plagal cadence.

Choose a chord per melody note. Working between the cadences, pick for each main melody note a primary chord that contains it, changing chord in a steady harmonic rhythm (often one chord per bar or per two beats), and preferring chord changes that flow well (I to IV, IV to V, V to I).

Keep the bass smooth. Use the chord roots in the bass, but choose octaves and the order of chords so the bass line moves reasonably smoothly rather than leaping wildly.

What markers reward: correct primary chords for the key, a chord chosen for each note that actually contains it, cadences planned at the phrase ends (perfect, imperfect or plagal), and a sensible harmonic rhythm. The strongest answers plan the cadences before filling in the middle.

Original6 marksA four-bar melody in G major ends its first phrase on A (the supertonic) and its second phrase on G (the tonic). Suggest a chord for each phrase ending, name the resulting cadence, and explain your choice.
Show worked answer →

First phrase ending on A (the supertonic): A is a note of chord V in G major (D, F sharp, A), so harmonise it with V (D major). Ending on V gives an imperfect cadence, open and unfinished, suiting the mid-point of the melody.

Second phrase ending on G (the tonic): G is the root of chord I (G, B, D). Approach it with V (D major) so the progression is V to I, giving a perfect cadence, conclusive, to close the melody.

Explanation: the chord must contain the melody note, and the cadence is chosen to match the melody's sense of pause (open) or close (final). An imperfect cadence at the half-way point and a perfect cadence at the end give the melody a natural comma-then-full-stop shape.

What markers reward: a chord that contains each ending note, the correct cadence names (imperfect for ending on V, perfect for V to I), and a clear reason linking the cadence to the melody's structure. Choosing a chord that does not contain the melody note loses marks.

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