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The problem it solves
Sometimes a triad sounds too bare, but a seventh already adds tension and a wish to resolve. The add9 and 6 chords give you a middle ground: they enrich the chord with one more note, yet stay stable and ask for no resolution.
Detailed theory
Key idea
The add9 adds the ninth (the second up an octave) to the triad: Cadd9 = C-E-G-D (1-3-5-9).
The sixth chord adds the sixth to the triad: C6 = C-E-G-A (1-3-5-6). Neither has a seventh.
Understand it
An added note is exactly that: you take a complete triad (root, third and fifth) and add one more note to give it colour, without removing any or turning it into a seventh chord.
The add9 adds the ninth, which is the second of the scale but an octave higher. In C, the ninth is D, so Cadd9 = C-E-G-D (1-3-5-9). Notice the third (E) is still there: that is why it is not a sus2.
The sixth chord adds the sixth of the scale to the triad. In C, the sixth is A, so C6 = C-E-G-A (1-3-5-6). Both chords keep the third, so they keep the major (or minor) quality of the starting triad.
The key point is that neither the add9 nor the sixth is a seventh. A seventh chord (like C7) adds the seventh degree and creates dominant tension; here there is no seventh, so the chord stays stable, rich and open, with no urge to resolve anywhere.
Think of it like adding a splash of colour to a plain wall: the wall (the triad) is still the same and still holds up, but the added note is the accent that gives it character. Remove the accent and it is the plain triad again.
Staff & keyboard
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C6 = C-E-G-A. The same C major triad with the sixth (A) added: a sweet, open colour, with no seventh.
How to recognise it
How it's written
In the symbol, add9 means "add the ninth to the triad" (Cadd9 = C-E-G-D) and a lone 6 means "add the sixth" (C6 = C-E-G-A). Since no 7 appears, you know they are not seventh chords.
How it feels
Play C major (C-E-G) first and then Cadd9 (C-E-G-D): you will hear the chord grow wider and brighter, yet just as stable. C the same with C6 (adding the A) and notice a sweet, open colour with no tension asking to resolve.
Common mistake
Confusing an add9 with a sus2: the sus2 removes the third, whereas the add9 keeps it and adds the ninth on top.
Thinking add9 or 6 are seventh chords: they are not, because they contain no seventh degree and create no dominant tension.
Try it
On the keyboard, play C-E-G and add the D on top: you have Cadd9. Remove the D and you are back at C major.
Play C-E-G and add the A: now you have C6. Compare the two colours, add9 and sixth, over the same triad.
On the instrument
Staff & keyboard
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Cadd9 = C-E-G-D. It is a complete C major triad (C-E-G) plus the ninth (D) added on top for colour.
Where it's used
- Enriching a triad
- Adding the ninth or the sixth to a major chord for a richer, more open colour without adding tension.
- A stable, modern ending
- Closing a phrase with a C6 or Cadd9 for a sweet colour that sounds stable, unlike a seventh chord that asks to resolve.
- Telling them from seventh and sus chords
- Recognising that the add9 and the sixth keep the third and have no seventh, unlike C7 and the sus2.
Examples
Chord progression
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Cadd9 → C6: two added-note colours over the same C major triad. Neither has a seventh nor asks to resolve.
Prepares you for
Exercises
Play add9 and 6 chords
Play the add9 or 6 chord shown, on any root.
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What does an add9 chord add to a triad?