Extended chords: 9th, 11th, 13th
Extended chords keep stacking thirds past the seventh: the 9th adds a second an octave up, the 11th a fourth, the 13th a sixth. A full C13 would contain seven notes — in practice nobody plays them all. Pianists routinely drop the fifth, and often the root too, keeping just the notes that carry the color.
Each extension has its own character. Ninths (C9, Cmaj9, Cm9) sound rich but smooth and appear all over soul and R&B. Elevenths usually show up over minor chords (Cm11) because the natural 11 clashes with a major third. Thirteenths are the full-color dominant sound of gospel and big-band jazz.
A practical way in: take a plain dominant seventh in a ii–V–I and add one note at a time. Play G7, then G9, then G13 resolving to Cmaj7. Voice the extensions near the top of the chord, and let the left hand cover just the root and seventh.
