9, 11, 13 — building extensions

Concept

Keep stacking thirds past the 7th: 9, 11, 13. A C9 chord is C E G Bb D (root, 3, 5, b7, 9). The 11 is the 4 (an octave up); the 13 is the 6 (an octave up). You don't have to play every note — guitarists often skip the 5 to make room.