guitar chord
B♭11
Six notes in theory; in practice the third is usually dropped, because the eleventh sits a half step above it.
Written the other way round it is A♯11 — the same twelve keys on a piano, a different piece of notation.
Every fingering that fits a hand
x11111
moderate · barre · no 3
x1111x
moderate · barre · no 3, 5
666768
moderate · barre
xddddd
moderate · barre · no 3
666868
moderate · barre · no 3
What is true of it
B♭11 is B♭, D, F, A♭, C and E♭ — 1, 3, 5, b7, 9 and 11 counted from B♭.
On an instrument that cannot reach all 6, the 3, 5 and 1 go first, in that order.
The easiest of the 5 fingerings below is x11111: one finger, one of them barring the 1st fret, held at the 1st fret.
None of them uses an open string, so this chord moves up the neck unchanged — the same shape 1 fret higher is the chord a semitone above.
It leaves out the 3. 6 strings cannot carry 6 notes, and the 3 is the one this chord can lose without changing what it is.
1 string stays silent.
Where its notes are on the neck
The brass dots are B♭, the root. Everything else is a chord tone with its degree printed in it.
With a different note underneath
B♭11/D
x50544
B♭11/F
111111
B♭11/A♭
43034x
An inversion is the same chord with a different note lowest. The name changes to a slash name because the bass is what a listener hears as the foundation.