Chord Alterations: b9, #11, #5, b13, and the alt Symbol

An altered chord is a dominant chord where one of the existing chord tones — usually the 5th or the 9th — has been raised or lowered by a semitone. For example, C7♭9 takes a C9 (C–E–G–B♭–D) and flattens the 9th to D♭, giving C–E–G–B♭–D♭. The chord does not gain a note; an existing note is modified. This distinguishes alterations from extensions (9, 11, 13), which add new notes on top of the stack. Altered chords create the dark, tense colors heard in jazz, fusion, and minor-key cadences.

C7♭9 is a specific alteration: the 9th is flattened and nothing else. C7alt is an umbrella symbol that means "use any combination of alterations" — the player chooses from ♭9, ♯9, ♯5, ♭5, and ♭13. A common realization of C7alt combines ♯5 + ♭9 (C–E–G♯–B♭–D♭), but the symbol does not mandate any single spelling. Use C7alt when you want freedom to improvise the color; use C7♭9, C7♯5, or C7♯11 when the arranger needs a specific sound.

The five most common alterations are ♭9 (flat the 9th), ♯9 (sharp the 9th), ♯5 (sharp the 5th), ♭5 (flat the 5th), and ♯11 (sharp the 11th). The ♭13 is enharmonically the same pitch-class as ♯5 in many contexts, so it is often treated as part of the ♯5 family. The 'alt' umbrella symbol (7alt) covers any combination of these. Of the five, ♭9 and ♯5 are the most common single alterations on dominant chords resolving to a minor tonic.

The classic home for an altered dominant is the V chord in a minor ii–V–i cadence — for example, Dm7♭5 → G7♭9 → Cm in the key of C minor. The alterations (especially ♭9 and ♯5) create maximum tension that resolves satisfyingly into the minor tonic. When the V resolves to a major tonic, a plain dominant 7th or a 9th usually sounds more natural; the altered colors are saved for the darker, more tense minor context. Altered dominants also appear in jazz fusion and post-bop as standalone colors on static dominant chords.