A Dominant Flat 9th

Fretboard

On the fretboard, A♯ represents B♭.

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About

The A7♭9 (A–C♯–E–G–B♭) alters the dominant 9 by lowering the ninth a half step. The B♭ creates a half-step clash with the root, amplifying the chord’s dissonance and intensifying its pull toward resolution. Because the interval between root and ♭9 is a minor ninth — one of the sharpest dissonances in tonal harmony — 7♭9 chords generate significantly more tension than standard dominants. They are especially idiomatic as V chords resolving to minor tonics; here A7♭9 → Dm, where the ♭9 relates diatonically to the target minor key. The 3rd and ♭7 remain the essential harmonic tones; the ♭9 is the defining alteration. The 5th can be omitted.