The problem it solves
Seventh chords (maj7, m7, 7) already sound rich, but when you want more density, brightness or a jazzy flavour you need notes above the seventh. Knowing where extensions come from and how they are written lets you build and read them with confidence.
Detailed theory
Key idea
Extensions continue the pattern of stacked thirds beyond the 7th: 9th, 11th and 13th (the 2nd, 4th and 6th raised an octave).
They add colour on top of the seventh chord without changing its basic function; in real voicings lower notes such as the 5th or the root are often omitted.
Understand it
A seventh chord already stacks four thirds: root, 3rd, 5th and 7th. Extensions simply keep going up the same path: add one more third and you reach the ninth (9), another the eleventh (11) and another the thirteenth (13). They are not strange new notes: they are the 2nd, the 4th and the 6th of the scale, raised an octave so they sit above the seventh.
Look at Cmaj9: C-E-G-B-D. You have the triad (C-E-G), the major seventh (B) and, on top, the D, which is the ninth (1-3-5-7-9). A dominant chord does the same with the minor seventh: C9 is C-E-G-Bb-D. And if you keep stacking up to the thirteenth, C13 adds the A (the 13th), the dominant chord most full of colour.
In practice you almost never play every note at once: with five, six or seven notes the hands cannot reach and the sound gets muddy. That is why real voicings omit notes: often the 5th drops (it adds little colour) and, if a bassist plays the root, you can even leave the root out on the keyboard. What never goes missing is what gives character: the 3rd, the 7th and the extension you have chosen.
Think of extensions as the upper floors you build on the foundation of a seventh chord: the ground floor (root, 3rd, 5th, 7th) holds the building up and gives it its function, and above it you add new rooms (9, 11, 13) that bring colour and breadth without touching the foundation.
There is an important colour exception: in a major chord the natural eleventh (the 4th) sits just a semitone above the major 3rd and clashes with it, making a harsh dissonance. That is why on major chords the 11th is usually raised to #11, which sounds open and bright instead of muddy. On minor chords, by contrast, the natural 11th coexists well with the minor 3rd.
Staff & keyboard
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A dominant C9: C-E-G-Bb-D. The same stack of thirds, but with the minor 7th (Bb) and the ninth (D) on top.
How to recognise it
How it's written
In chord symbols, the number says how far the stack goes: maj9, 9, 11 and 13 indicate the highest extension included. Cmaj9 is major triad + major 7th + 9th; C9 (without "maj") is a dominant with minor 7th + 9th; C13 is a dominant up to the thirteenth. An accidental before the number (for example #11 or b9) marks the altered extension.
How it feels
Play the triad first, then the seventh and finally add the extension: you will hear how each new layer opens and colours the sound without changing which note is the root. The 9th sounds bright and fresh; the #11 gives a floating air; the 13th, over a dominant, fills the chord with warm colour.
Common mistake
Putting the natural 11th in a major chord: it clashes with the major 3rd a semitone away and sounds harsh; on major chords it is raised to #11.
Trying to play every note at once; with 5-7 notes the sound gets muddy and the hands cannot reach. Omit the 5th or the root and keep the 3rd, the 7th and the extension.
Try it
Build Cmaj9 by stacking thirds from C: C-E-G-B-D, and check that the D is the 2nd raised an octave (the ninth).
Play a C13 omitting the 5th and the root (leave E-Bb-A-D): you will feel it still sounds like a dominant full of colour even with notes missing.
On the instrument
Staff & keyboard
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Cmaj9 stacked in thirds: C-E-G-B-D (1-3-5-7-9). The top D is the ninth that colours the triad with a major seventh.
Where it's used
- Richer keyboard voicings
- Adding the 9th or 13th to a seventh chord to open its sound, omitting the 5th or the root when needed.
- Jazz colour
- Turning a basic ii-V-I into Dm9 → G13 → Cmaj9 to give density and stylistic flavour to the progression.
- Choosing the right tension
- Knowing when to raise the 11th to #11 on major chords to avoid the clash with the major 3rd.
Examples
Example with rhythm
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Cmaj9 stacked in thirds as a full chord: C-E-G-B-D (1-3-5-7-9). The top D, highlighted, is the ninth: the note that keeps the stack going above the major seventh and colours the triad without changing its root.
Example with rhythm
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A ii-V-I with extensions as a chorale: Dm9-G13-Cmaj9. Each chord stacks colour above the seventh — the 9th over the ii, the 13th (the E) that colours the dominant and the 9th over the I — without changing its function. The colour notes are highlighted.
Generate a new example
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Generate fresh progressions with extensions in any key (ii9–V13–Imaj9 transposed). Press for a new one and hear the colour the ninth and thirteenth add.
Exercises
Recognise chords with extensions
Chords with a ninth and a thirteenth (maj9, 9, m9, 13) on top of the seventh.
Complete 10 attempts · 70% accuracy to pass
Mini test
Check that you've got it.
0/10 answeredQuestion 1/10
What are the extensions of a chord?