Chord Extensions

Extensions are notes stacked in thirds beyond the seventh: the 9th, 11th, and 13th. They add color and complexity to seventh chords without changing fundamental chord function.

Natural Extensions

A 9th is a compound 2nd (14 half steps above root). A 11th is a compound 4th (17 half steps). A 13th is a compound 6th (21 half steps).

A full 13th chord contains all seven notes of the parent scale: R-3-5-7-9-11-13. In practice, you rarely play all seven; instead, you select the extensions that add the desired color and omit redundant tones (often the 5th and sometimes the root).

Altered Extensions

Altered extensions raise or lower natural extensions by a half step. These are most commonly applied to dominant 7th chords:

• b9 and #9: Create intense dissonance. The "Hendrix chord" (7#9) is one of the most iconic sounds in music.
• #11: Adds a Lydian quality. Very common on major 7th and dominant chords.
• b13: Darkens the chord. Combined with b9, creates the full altered dominant sound.
• b5 and #5: Technically altered 5ths, often grouped with extensions.

Extensions by Chord Type

Maj7: Natural 9, #11, 13 are common. Avoid natural 11 (clashes with the 3rd). Min7: Natural 9, 11, 13 all work beautifully. 13 adds a Dorian flavor. Dom7: Anything goes. Natural 9/13 for "inside" sound; b9, #9, #11, b13 for tension. The dominant chord is the most flexible chord in jazz. m7b5: Natural 9, 11, b13 (natural 11 is characteristic).

Voicing Extensions

On guitar, you typically include the root (or let the bass player handle it), the 3rd, the 7th, and one or two extensions. The 3rd and 7th define the chord's quality and function; the extensions add color. This is why shell voicings (root-3-7) are the foundation: you add extensions on top of a shell.

Notation

"C9" means C7 with a natural 9 (implies all lower extensions). "Cadd9" means C major triad with a 9 but no 7th. "C7b9#11" explicitly names which extensions are altered.