Altered and quartal chords
A reference card, and the end of the chord road. Everything so far has stacked thirds. The 20th century broke that habit — altering the notes, stacking different intervals, or piling chords on top of each other. You'll see these on jazz charts and in modern scores. Recognise the idea; don't drill it.
Altered dominants
A dominant 7th chord with one or more of its colour notes bent — the 5th or the 9th raised or lowered:
| Symbol | The alteration | Sound |
|---|---|---|
| 7♭5 / 7♯5 | 5th flattened / sharpened | unstable, "outside" |
| 7♭9 | 9th flattened | dark, Spanish-tinged |
| 7♯9 | 9th sharpened (the "Hendrix chord", E7♯9) | crunchy, bluesy-angry |
| 7alt | everything altered at once | maximum tension before resolution |
They still do the dominant's job — pull to the tonic — just with more grit on the way.
Quartal harmony — stacking 4ths
Instead of thirds, stack perfect 4ths: C–F–B♭, or D–G–C. The result has no clear major/minor identity — it's open, modern, a little cold. McCoy Tyner built a piano style on it; film composers use it for "space" and "technology."
Polychords and clusters
- Polychord — two triads stacked and played at once (a C major over an F♯ major, say). Stravinsky's Petrushka chord is the famous one. The effect is two keys colliding.
- Cluster — a fistful of adjacent notes (C–D–E–F all together). Used for noise, impact, or texture rather than harmony.
Where you land
If a chart or a score throws 7♯9, a stack of 4ths, or two triads at once — recognise it as 20th-century / jazz harmony: the old "stack thirds, resolve dissonance" rules have been loosened or dropped. As a single-line player you read it for the bass note and the mood, and carry on.