Extended chords go beyond the seventh by stacking more thirds - adding 9ths, 11ths, and 13ths.
Extended chords add upper chord tones and therefore more frequency and interval content. The extra notes can add colour, but their register and spacing matter: a crowded voicing may obscure the root or clash with the melody.
How extensions stack
Extensions continue the pattern of stacking thirds above the seventh. The names come from counting scale degrees from the root.
The 9th is the same note as the 2nd, an octave up. The 11th = 4th up an octave. The 13th = 6th up an octave.
9th chords
A 9th chord is a seventh chord with the 9th added. The added note can widen the voicing and introduce extra colour, especially when the register leaves space between the chord tones.
11th and 13th chords
The further you extend, the more notes you're layering. In practice, producers often leave some notes out to keep things clean - you don't need every chord tone in the full stack.
Notice how C11 and C13 above drop the 5th. This is normal - in extended chords, the 5th is often the first thing to go because it doesn't add much character.
"Add" chords vs full extensions
A Cadd9 is different from a C9. Cadd9 adds the 9th to a triad (no seventh). C9 includes the seventh.
Try it
Build Cmaj7 first, then add D to make Cmaj9. Now remove G and compare the clean voicing with the fuller version. Notice which notes define the chord and which note can be left out without losing its colour.
Extensions add colour, but a useful voicing does not need every possible note. Listen for clarity as well as richness.
When to use extensions
Extended chords add sophistication. A simple triad progression can sound completely different with 9ths and 7ths layered in. But more isn't always better - sometimes a plain triad is exactly what the track needs.
Key takeaway
Extensions (9th, 11th, 13th) stack more thirds above the 7th. A 9th is a useful first extension to hear. You can omit or redistribute notes to keep a voicing clear, while "add" chords include the named note without implying a 7th.
Next: what happens when you replace the third entirely - suspended chords.
Experiment with extended chords
Starts can generate 7th and 9th chord voicings in any key.