The 11 of B♭ is E♭
The 11 (eleven) of B♭ (B flat, Bb) is E♭ (E 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: B, C, D, E.
Then the sound: a perfect eleventh is 17 semitones above B♭, and the E that is 17 semitones up is E♭.
That is why it is E♭ and not D♯: D♯ is the same key on the piano, but from B♭ 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 B♭m11:
B♭ · D♭ · F · A♭ · C · E♭
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 B♭
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