Augmented ninth

The augmented ninth (A9) spans 15 semitones. It is the compound form of an augmented second and carries a bright, biting tension often used in altered harmony.

Construction and spelling

A9 is built as an octave plus augmented second, for example C-D# above the octave. Even when it may resemble m3 by pitch in equal temperament, spelling marks ninth-function behavior. This distinction is crucial in harmonic reading.

Harmonic and melodic usage

Harmonically, A9 appears prominently on altered dominant chords and blues-influenced sonorities. Melodically, it creates sharp expressive color and directional pull. It is effective when you want tense brilliance rather than neutral extension.

Examples

• #9 color in dominant-function voicings
• Blues/jazz sonorities mixing major and minor third colors
• Chromatic lines resolving tensions