Bb11 is Bb, F, Ab, C and Eb — with the eleventh added, and the third usually dropped.
It sounds suspended rather than tense. The eleventh sits a semitone above the major third, so the third is usually dropped; some shapes here keep it anyway.
Dominant — but effectively a sus chord with a ninth.
Also written Bb9sus4 or Bb dominant 11.
Bb11 has the notes of the ii over the V’s root, so it can take the ii’s bar: Bb11, then Bb9, then Eb. The eleventh gives way to the third, and the V resolves as usual.
Listen for: The eleventh falls a half step to the third, then rises a half step into the root of the I.
In which keys. Every note of Bb11 belongs to C minor, C# major, Eb major, F minor, Ab major and Bb minor.
Every note of Bb11 is in Bb natural minor, Bb dorian and Bb mixolydian.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where Bb11 sits in it. Every scale.
Change one note of Bb11 and you have (drop the flat seventh), (the eleventh becomes the minor third), (add the minor third), (drop the ninth) or (the eleventh becomes the major third).
Bb11 is a dominant chord — root, major third, fifth, minor seventh. Dominant seventh chords on every root.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.