The 11 of A♭ is D♭
The 11 (eleven) of A♭ (A flat, Ab) is D♭ (D flat): a perfect eleventh above the root.
How to find it
An 11th is a 4th an octave up, so it takes the 4th's letter: A, B, C, D.
Then the sound: a perfect eleventh is 17 semitones above A♭, and the D that is 17 semitones up is D♭.
That is why it is D♭ and not C♯: C♯ is the same key on the piano, but from A♭ it would be a 10th, not an 11th.
Where you meet it
The 11 is the 4 an octave up. It sits naturally on minor chords; on a major chord it clashes with the 3rd, which is why the ♯11 exists.
In A♭m11:
A♭ · C♭ · E♭ · G♭ · B♭ · D♭
The 11 in every key
| Key | The 11 |
|---|---|
| C | F |
| G | C |
| D | G |
| A | D |
| E | A |
| B | E |
| F♯ | B |
| D♭ | G♭ |
| A♭ | D♭ |
| E♭ | A♭ |
| B♭ | E♭ |
| F | B♭ |
Every degree of A♭
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