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What Is a Chord Extension?

What Is a Chord Extension?

Extensions are chord tones stacked beyond the seventh: the 9th, 11th, and 13th. Keep stacking thirds past a seventh chord and you pass 1-3-5-7 into 9, 11, 13 — which are just the 2nd, 4th, and 6th scale notes an octave up. C9 is a C7 with a D added; C13 adds the A too. Extensions add color, not function — a C13 still behaves like C7, just dressed better.

Reading the numbers

  • C9, C11, C13 — dominant sevenths plus extensions (the 7 is implied and flat).
  • Cmaj9, Cm9 — the same idea on major-seventh and minor-seventh chords.
  • Cadd9 — no seventh, just the 9 added to a plain triad. "Add" means "skip the stack, just add this note." The jangly add9 is all over acoustic pop.
  • Csus2/Csus4 — not extensions: the 2 or 4 replaces the third — see Sus Chords Explained.

The complete symbol decoder is Guitar Chord Symbols Explained.

Extensions on guitar

Six strings can't hold a full 13th chord (that's seven notes), so guitarists edit: keep the defining tones — the third and seventh — add the extension, and drop the fifth or even the root (the bass player has it). This is why jazz chord shapes look sparse: they're the essential four notes of a big chord, voiced for maximum color per finger.

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