Altered and Extended Chords

Extended chords go beyond the seventh by continuing to stack thirds above the chordal root: ninths (9), elevenths (11), and thirteenths (13) expand the basic triad or seventh chord into richer, more colorful sonorities. A C dominant thirteenth chord, for example, contains all seven distinct pitch classes of C major arranged in thirds from C to A. In practice, not all tones are sounded simultaneously — composers and performers select the most harmonically meaningful notes and omit the rest, typically keeping the third, seventh, and the extensions that give the chord its specific character.

Altered chords modify one or more of a dominant seventh chord's diatonic extensions by raising or lowering them a half step. The most common alterations involve the fifth (♭5 or ♯5), the ninth (♭9 or ♯9), and the eleventh (♯11, also called the Lydian dominant tone). These alterations dramatically increase the chord's harmonic tension — the ♭9 adds a grinding half step against the root, while the ♯11 introduces the tritone into what was already the most dissonant diatonic chord. Altered dominants appear prominently in jazz, late Romantic music, and film scores, where heightened tension before resolution is desirable.

The ♭9 chord has special historical significance in chromatic tonal music: when a German augmented sixth chord is respelled as a dominant seventh with a ♭9 (without its root, or over a different bass note), the two chords reveal a deep enharmonic relationship. This equivalence allowed composers to modulate between keys using common-tone or common-chord logic while exploiting the tonal ambiguity of both sonorities.

In analysis, extended and altered chords are labeled by stacking the relevant numerals above the Roman numeral (e.g., V♭⁹, V♯¹¹, or simply "alt" in jazz notation). When encountering a dense chromatic chord, first identify the root and quality of the underlying triad or seventh chord, then catalog the additional tones and note any half-step alterations from the diatonic norm. The voice-leading into and out of these chords is usually smooth — the additional chromatic notes tend to resolve by half step, making these harmonies coloristic intensifications of familiar functional progressions.