Extended chords keep stacking notes above a seventh chord, past the octave. The 9th is the second note of the scale placed an octave higher, the 11th is the fourth and the 13th is the sixth. Starting from C7 (C–E–G–B♭), adding D gives C9; adding D and A gives the dominant 13th sound. In chord-symbol shorthand a 9, 11 or 13 implies that the seventh is present too, so Cmaj9 means C–E–G–B–D.
Not every extension suits every chord. A plain 11th (F over C) clashes with the major third (E), so on major and dominant chords players usually leave it out, while on minor chords such as Cm11 (C–E♭–G–B♭–D–F) it sounds smooth. In practice musicians drop some notes, often the fifth or even the root, to keep the voicing clear. Ninth and eleventh chords played on an electric piano or a sampled jazz record are the harmonic fingerprint of jazz lo-fi: dense enough to sound sophisticated, soft enough to stay in the background.