Melody, harmony and the triad
Melody is notes one after another, like words in a sentence. Harmony is notes sounding at the same time, like a choir singing different notes together.
A chord is three or more notes played together. The simplest chord is a triad. Start on any note (the root), skip one letter to get the third, skip again to get the fifth. From C: C–E–G.
Semitones and intervals
A semitone is the smallest step on a keyboard: from one key to the very next key, black or white. An interval is the distance between two notes. A major third is 4 semitones; a minor third is 3; a perfect fifth is 7.
Consonance and dissonance
Consonant intervals (octave, fifth, thirds, sixths) sound stable. Dissonant ones (seconds, sevenths, the tritone of 6 semitones) sound tense and want to move on. Harmony is a play of tension and release.
Major, minor and the chords of a key
A major triad = root + 4 semitones + 3 semitones (C–E–G). A minor triad = root + 3 + 4 (A–C–E). Two less common triads are diminished (3+3, B–D–F) and augmented (4+4, C–E–G#).
Build a triad on each note of a major scale and number them with Roman numerals. Capital = major, small = minor: I, ii, iii, IV, V, vi, vii°. In C major: C, Dm, Em, F, G, Am, B°.
Functions: home, away, pull
- Tonic (I): home, rest.
- Subdominant (IV, also ii): moves away from home.
- Dominant (V, also vii°): tension that pulls back to I. Its third is the leading note (B in C major), one semitone below home.
Adding a fourth note a third higher makes a seventh chord. V7 (G–B–D–F) pulls home even more strongly.
Progressions, cadences and harmonising a melody
A chord progression is a chain of chords. Common ones: I–IV–V–I, I–V–vi–IV (many pop songs), ii–V–I (jazz).
Four cadences
- Perfect (authentic): V–I. Sounds finished.
- Plagal: IV–I. A soft "Amen" ending.
- Imperfect (half): ends on V (e.g. I–V). Sounds like a comma, not a full stop.
- Interrupted (deceptive): V–vi. You expect home and get a surprise.
Harmonising a tune: simple steps
- Find the key.
- For each strong beat, list chords that contain the melody note (C fits I, IV or vi in C major).
- Choose so the chords make a sensible path and end with a cadence.
- Move the inner voices by the smallest step (good voice leading). Avoid two voices moving in parallel fifths or octaves in classical style.
An arrangement gives these chords to instruments: bass plays roots, a guitar or keyboard plays the chord, the voice sings the tune.
Counterpoint, canon, modes and how harmony changed
Counterpoint is writing two or more tunes that are each interesting on their own but fit together. Good counterpoint uses contrary motion (one voice goes up while the other goes down) and mostly consonant intervals on strong beats.
A canon (or round) is one tune that is imitated by another voice starting later. A fugue is a bigger, stricter piece built on imitation.
Modes
Before major and minor took over, music used modes: scales using the white keys from different starting notes. Dorian (D to D) and Mixolydian (G to G) still appear in folk, film and rock music. Indian ragas also use fixed sets of notes, but they are mainly melodic, over a drone, not chord-based.
A short history
- Medieval and Renaissance: modal music and counterpoint.
- Baroque to Romantic (about 1600–1900): functional harmony in major and minor keys, growing richer with chromatic chords.
- 20th century onward: extended chords (9ths, 11ths) in jazz, free dissonance, and simple loop-based harmony in pop.
Try it
On the 3D keyboard, build C major and C minor. Only the middle note moves, by one semitone, yet the mood changes. Then hum a melody and test which of I, IV or V fits each bar.
Key formulas and definitions
- Triad = root + third + fifth (skip a letter each time)
- Major triad: 4 + 3 semitones · Minor triad: 3 + 4 · Diminished: 3 + 3 · Augmented: 4 + 4
- Major key chords: I ii iii IV V vi vii°
- Tonic = I (home) · Subdominant = IV (away) · Dominant = V (pull)
- Perfect V–I · Plagal IV–I · Imperfect …–V · Interrupted V–vi
Worked examples
1. Build the triad on D. Is it major or minor in the key of C major?
Skip letters from D: D–F–A. D to F = 3 semitones, F to A = 4. Gaps 3 + 4, so it is minor: chord ii (Dm).
2. Name the primary chords of G major and their notes.
G major scale: G A B C D E F#. I = G–B–D, IV = C–E–G, V = D–F#–A.
3. A phrase ends with chords G then C in C major. Which cadence is it?
G is V and C is I. V–I is a perfect (authentic) cadence.
4. The melody note on beat 1 is E in C major. Which primary chord fits?
Look for chords containing E: I (C–E–G) has it; IV (F–A–C) and V (G–B–D) do not. Choose I.
5. A song in C uses C – G – Am – F. Write it in Roman numerals.
C = I, G = V, Am = vi, F = IV. So I–V–vi–IV, a very common pop progression.
Common mistakes
- Counting letters instead of semitones. C–E and D–F both skip a letter, but C–E is 4 semitones and D–F is 3.
- Thinking every chord in a major key is major. ii, iii and vi are minor; vii° is diminished.
- Mixing up perfect and imperfect cadences. Perfect ends on I; imperfect ends on V.
- Thinking counterpoint means chords only. It means independent tunes that fit together.