Also written A♯11 — the same pitches spelled the other way.
The B♭ Dominant 11th chord and everything around it — the notes and how they are built, the shape on piano and guitar, the sound, and the keys it belongs to.
B♭11 is Bb, F, Ab, C, Eb — the 1, 5, b7, 9 and 11 above the root B♭.
Root notes are red. There is no standard open shape for this chord — any group of these notes on the neck sounds the chord.
Every position of the chord tones on the ukulele neck — any group that sounds only Bb, F, Ab, C, Eb plays the chord.
B♭11 is a diatonic chord in these keys:
Work out the chords of a whole key with the Scale & Chord Finder, name a chord from its notes with the Chord Identifier, or browse every chord tool. The theory behind how chords are built is in the free Cloud Atelier music guides.