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Extended chords add intervals beyond the seventh to a chord's basic structure: the ninth (an octave plus a second above the root), the eleventh (an octave plus a fourth), and the thirteenth (an octave plus a sixth). These chords are built by stacking diatonic thirds above a seventh chord, producing rich sonorities characteristic of jazz, impressionism, and late Romantic harmony. In practice, not all chord tones are voiced simultaneously — the eleventh is often omitted from major chords because it clashes with the third, and the fifth is routinely dropped to keep voicings manageable. The ninth can be major (natural), minor (flat nine), or augmented (sharp nine), each with a distinctive color.
Build extended chords from the root up at the keyboard, naming each interval as you add it. Then practice jazz voicings that omit the root and fifth, focusing on the third, seventh, and the extended tone. Listen to jazz piano recordings (Bill Evans, McCoy Tyner) to hear how extended chords are used in context.
You've built seventh chords — triads with a seventh stacked above the root, creating four-note sonorities that carry new tension and color. You understand intervals as measured diatonic distances. Extended chords simply continue the stacking process: if a seventh chord is a triad plus a third on top, a ninth chord is a seventh chord plus one more third, and so on up the diatonic stack. The interval names reflect the total distance from the root — the ninth is one octave plus a major or minor second, the eleventh is one octave plus a perfect fourth, the thirteenth is one octave plus a major or minor sixth.
Think of the interval numbers as a spiral rather than a straight line. After the octave (8), we don't restart at 1 — we continue: 9 is the same pitch class as scale degree 2, 11 is the same pitch class as scale degree 4, 13 is the same pitch class as scale degree 6. The distinction matters because the number signals the note's function. In a Cmaj9 chord (C–E–G–B–D), the D functions as a ninth — an extension above a complete seventh chord — not as a simple second within a triad. The context of the full chord changes how the note is heard and how it should be voiced.
The practical reality is that these chords are rarely voiced with all their theoretical notes. A C13 chord contains seven distinct pitches (C, E, G, B♭, D, F, A) — one per voice in a full orchestra, but far too many for two hands at a piano. Jazz practice developed shell voicings to solve this: drop the root (the bassist covers it), drop the fifth (acoustically redundant in most contexts), and keep the third (defines major/minor quality), the seventh (defines dominant/major 7th/minor 7th quality), and the extension that gives the chord its color. This produces a compact 3–4 note voicing that implies the full extended chord without crowding any register.
The most important expressive distinctions involve the ninth. A major 9th (♮9) is warm and open — characteristic of major seventh chords and lush Romantic harmony. A flat 9 (♭9) places a half-step crunch above the root, giving dominant seventh chords a tense, dissonant quality used in flamenco, jazz, and late Romantic chromaticism. The sharp 9 (#9) produces a collision: the #9 sounds like a minor third above the octave while the chord's major third is also present, creating an ambiguous major-minor clash heard in blues, rock, and funk. The "Hendrix chord" (E7#9) is the most famous example. These three flavors of the ninth alone give the dominant seventh chord enormous expressive range within a single harmonic function.
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