Harmony is musical magnetism. Chords draw the ear towards other chords, and the sound flows in a direction that makes sense to a listener even if they have never studied a note of theory. Understanding why lets you write progressions that pull exactly where you want - and surprise when you want.
Consonance and dissonance
Two factors decide how notes sound together:
- How compatible their frequencies are. Notes whose frequencies form simple ratios - an octave (2:1), a fifth (3:2), a fourth (4:3) - sound stable and blended. This is consonance.
- The harmonics they contain. Every real note includes overtones above its fundamental. How those overtones line up - or clash - depends on the sound's timbre, which is why the same chord can sound smooth on a pad and harsh on a distorted guitar.
Dissonance is the opposite: notes whose frequencies clash, creating roughness and tension. Dissonance is not bad. It creates the desire to return home - and that desire is what moves music forward.
Tension and release
Every key has a home: the tonic, or key centre. In C major, the C major chord is home - the I chord.
Some chords pull towards home more strongly than others. The strongest is V, the chord built on the fifth: in C, G major. G contains B, a semitone below C, and F, a semitone above E. Both want to resolve - B up to C, F down to E - and the result is a powerful magnetic pull back to C. Add the seventh (G7) and the pull is stronger still.
The shape of most tonal music is a cycle of moving away from home, building tension, and returning: I to IV (moving away), to V (tension), and back to I (release).
Cadences
A cadence is the chord movement at the end of a phrase.
- Perfect (authentic) - V to I. Final. Full stop.
- Plagal - IV to I. Softer - the "amen" at the end of a hymn.
- Imperfect (half) - ending on V. A comma, not a full stop. The phrase is not finished.
- Interrupted (deceptive) - V to vi. The ear expects I and gets the relative minor. A built-in surprise.
The modes
Play the notes of C major starting from each of its seven notes and you get seven modes. Each has the same notes but a different home - and a distinctly different colour.
- Ionian (C to C) - the major scale. Bright, settled.
- Dorian (D to D) - minor with a raised sixth. Soulful, cool, slightly hopeful. Funk, jazz, a great deal of house.
- Phrygian (E to E) - minor with a lowered second. Dark, Spanish-sounding, tense. Flamenco, metal, some techno.
- Lydian (F to F) - major with a raised fourth. Dreamy, floating, wondrous. Film scores.
- Mixolydian (G to G) - major with a lowered seventh. Bluesy, laid-back, rock and roll.
- Aeolian (A to A) - the natural minor scale.
- Locrian (B to B) - diminished, unstable; rarely used as a home.
Modes are easiest to hear over a drone. Hold a low D and play the white notes from D to D: that is Dorian. Hold an F and play F to F: Lydian. The note that differs from the plain major or minor scale is the one that gives each mode its character - make sure your melody uses it.
Modal interchange
Modal interchange - also called borrowing - takes a chord from a parallel mode or key (same home note, different scale) and drops it into your progression.
- In C major, borrowing from C minor gives you Fm (iv), Ab (bVI) and Bb (bVII). C-Ab-Bb-C is the heroic sound of countless rock and film themes.
- The minor iv in a major key (C-F-Fm-C) is one of the most emotional moves in pop.
- The Picardy third goes the other way: ending a minor-key piece on a major tonic for a moment of unexpected light.
Reading a progression that changes key
Take one common reading of the chords of "Killing Me Softly":
- Bar 1 - Em7, Am7, D, Gmaj7. Simplified: Em, Am, D, G. In numerals: i - iv - VII - III. Conclusion: E minor.
- Bar 2 - Em7, Am7, D, C9. Still i - iv - VII - VI: still E minor.
- Bar 3 - G, C, F... then E major.
There is no F chord in E minor, so something has happened. G to C to F is a little journey in C major. Then F slides down a semitone to E - and E arrives as a major chord, a Picardy third, instead of the Em the song began with.
Two ways to explain that F, both useful:
- F is the bII of E - a chord a semitone above the tonic, often called a Neapolitan in classical music.
- It is also a tritone substitution. The normal dominant chord leading to E would be B. F is a tritone away from B and shares its most important tension notes, so it can stand in for it - approaching E from a semitone above instead of a fifth.
The listener hears none of this terminology - only a progression that seems to wander off and then lands somewhere warmer than expected. That is harmony doing its job.
Do this now
- Over a sustained low D, improvise a melody using only white notes: you are in D Dorian. Then do the same over E (Phrygian) and F (Lydian).
- Choose any mode - Dorian, Lydian, Mixolydian or Phrygian - and write a riff or melody that uses most of its notes, especially the characteristic one. Add chords if you want.
- In your C major project, replace one F chord with Fm. Then try ending the progression C-Ab-Bb-C.