guitar chord
B♭dim7
Three minor thirds stacked. The seventh is doubly diminished, which is why it is written with a double flat.
Written the other way round it is A♯dim7 — the same twelve keys on a piano, a different piece of notation.
Every fingering that fits a hand
x12020
open
645050
moderate
x1202x
open
x12023
moderate
64505x
hard
What is true of it
B♭dim7 is B♭, D♭, F♭ and A♭♭ — 1, b3, b5 and bb7 counted from B♭.
A♭♭ is a double flat, and it has to be: every letter from B♭ upwards is used once, so the bb7 cannot be written any other way.
The easiest of the 5 fingerings below is x12020: 3 fingers, held at the 1st fret.
2 of them use open strings, which is what makes a shape quiet to change to.
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♭dim7/D♭
x45350
B♭dim7/F♭
012020
B♭dim7/A♭♭
312020
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.