Extended chords add thirds above the seventh — the ninth, eleventh, and thirteenth. They are the harmonic foundation of jazz, bossa nova, and late Romantic orchestral writing, and they require trained ears to hear clearly.
BUILDING BEYOND THE SEVENTH
Western harmony, as taught in most undergraduate curricula, ends with seventh chords. But composers and jazz musicians regularly use chords that extend further up the tertian stack: ninth chords (root-3rd-5th-7th-9th), eleventh chords (adding the 11th), and thirteenth chords (adding the 13th, which is a sixth above the root at the octave). In jazz notation, these extensions are written as numerals after the chord symbol: Cmaj9, Dm11, G13.
The ninth is a second displaced an octave: C9 adds D above the seventh. The eleventh is a fourth displaced: C11 adds F. The thirteenth is a sixth displaced: C13 adds A. These additions create chords of extraordinary richness and complexity. A full thirteenth chord has seven notes — one for every diatonic scale degree — and represents the entire scale harmonized into a single chord.
In practice, extended chords are rarely played with all their tones. Jazz pianists typically omit the fifth (the least important voicing tone) and often the root (left to the bass player), voicing the chord as a cluster of the defining extensions. A G13 chord might be voiced as F-B-E-A in the right hand, with the bassist playing G — the chord's identity comes from the interaction of these extensions.
MAJOR NINTH, MINOR NINTH, DOMINANT NINTH
The three most common ninth chord types reflect the underlying seventh chord types. Cmaj9 (major ninth chord) = C-E-G-B-D: lush, stable, dreamy — the sound of early morning light in countless film scores. Cm9 (minor ninth chord) = C-Eb-G-Bb-D: dark and rich, the sound of sophisticated minor harmony in jazz ballads. G9 (dominant ninth chord) = G-B-D-F-A: the dominant seventh with an added ninth, the most common extension in jazz harmony. When the ninth is raised (G7#9 = G-B-D-F-A#), the so-called 'Hendrix chord' appears — used famously in Purple Haze and creating a striking minor-on-major clash.
HEARING EXTENDED CHORDS
Extended chords can initially sound complex and undifferentiated to untrained ears — everything sounds 'jazzy' without specific identification. Developing the ability to distinguish Cmaj7 from Cmaj9 from Cmaj9#11 requires exposure and focused listening. The recommended approach: first master seventh chord recognition thoroughly. Then add nines: listen for the added tone above the seventh — does it sit a whole step above the root? Then add the character of the extension (natural 9, #9, b9).
The ♯11 (Lydian dominant) is a particularly distinctive sound: in a dominant seventh context, the raised eleventh (#11) is the tritone above the root, creating a super-bright, slightly unsettled quality that is the defining colour of the Lydian dominant scale (Mixolydian #4). It is ubiquitous in film scores, jazz piano voicings, and fusion.
ALTERED DOMINANT CHORDS
The altered dominant — a dominant seventh chord with one or more altered upper extensions (b9, #9, b13, #11) — is a defining sound of bebop and modern jazz. The fully altered dominant contains both b9 and #9 (also called the "Hendrix chord" component), as well as #11 and b13. The corresponding scale, the altered scale (or super-Locrian), provides the melodic vocabulary for improvising over these chords.
In a ii-V-i progression in minor, the V chord is almost always altered. The half-diminished ii (Dø7 in C minor) resolves to an altered G7 (G7alt — containing the altered upper extensions), which then resolves to the minor tonic Cm. The voice leading involves chromatic motion that is one of the signature sounds of jazz harmony.
For ear training, altered dominants have a distinctly sharper, more dissonant quality than unaltered dominants. The presence of the b9 (which sits a half step above the root) and the #9 (a minor third above the root, creating a major/minor clash with the natural 3rd) gives the chord its characteristic spiky quality.
VOICINGS AND HARMONIC RHYTHM
In jazz piano playing, the voicing of an extended chord — which notes are played and in what order — is as important as the chord's basic identity. The same Cmaj9 chord can be voiced dozens of different ways: in close position (notes packed together), in open position (notes spread across registers), in rootless voicings (omitting the root), in shell voicings (just root, third, and seventh), or in cluster voicings (adjacent notes creating dissonance).
Professional jazz pianists like Bill Evans, McCoy Tyner, and Herbie Hancock developed distinctive voicing approaches that became style markers. Evans popularised rootless voicings that emphasized the 3rd, 7th and extensions, and used fourth-based voicings on modal tunes like So What. Tyner used widely-spread quartal voicings in the left hand with melodic lines in the right. Hancock combined chromatic voice leading with open-position voicings for a modern, slightly impressionist sound.
DEBUSSY AND THE ORIGIN OF EXTENDED HARMONY
While jazz musicians popularized extended chords in 20th-century practice, the harmonic language has earlier roots. Claude Debussy's music (1862-1918) uses extended chords pervasively: ninth chords, eleventh chords, thirteenth chords, and chords with added notes (sixths, ninths) appear throughout his piano and orchestral works. La Mer, Prélude à l'après-midi d'un faune, and the piano preludes all demonstrate extended harmony used for colour rather than function.
Debussy's influence on jazz harmony has been documented extensively. Bill Evans openly cited Debussy as a model; the voicing techniques of cool jazz pianists derive directly from Debussy's piano writing. Maurice Ravel's harmonic language has similar influence, with his orchestral colours and extended chords providing a template for jazz arrangers like Gil Evans.
UPPER STRUCTURE TRIADS
A powerful jazz voicing technique is the upper structure triad: playing a triad in the right hand whose notes function as upper extensions of a chord whose root is supplied by the bassist (or assumed). For example, over a C7 chord, playing a D major triad (D-F#-A) in the right hand produces a C13#11 sound — the D triad contains the 9, #11, and 13 of C7. Mapping triads onto chord extensions provides a quick way to access complex harmonic colours.
Pianists such as Mulgrew Miller used upper structure triad voicings extensively. The technique appears throughout modern mainstream jazz and provides a useful framework for understanding extended chord voicings as combinations of more familiar units.
DISTINGUISHING EXTENDED CHORDS BY EAR
For ear training purposes, focus first on identifying the underlying chord type (major 7, dominant 7, minor 7). Then add awareness of extensions: the 9 sits a major or minor 9th above the root; the 11 sits a perfect 11th above; the 13 sits a major 13th above. Altered extensions (b9, #9, #11, b13) have characteristic dissonant qualities that distinguish them from natural extensions.
Use jazz play-along recordings (Aebersold series, iReal Pro) to practice identifying chord extensions in real musical context. Listen to recordings of Bill Evans, Herbie Hancock, and Brad Mehldau for examples of sophisticated extended-chord voicings used in standard tunes.
SOURCES & FURTHER READING
Open Music Theory — Extended and Altered Chords (openmusictheory.net, CC BY-SA 4.0). Russell, George. The Lydian Chromatic Concept of Tonal Organization. First published 1953. — The theoretical foundation for modern jazz harmony and chord-scale theory. Levine, Mark. The Jazz Theory Book. Sher Music, 1995. — Comprehensive practical guide to jazz harmony including extended chords. teoria.com — Advanced Chord Exercises. Benward and Saker. Music in Theory and Practice, Vol. II. McGraw-Hill.