piano chord
B♭dim7
B♭1D♭♭3F♭♭5A♭♭♭♭7
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.
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.