piano chord
B♭dim
B♭1D♭♭3F♭♭5
Two minor thirds stacked. The fifth is diminished, so it cannot be dropped without losing the chord.
Written the other way round it is A♯dim — the same twelve keys on a piano, a different piece of notation.
What is true of it
B♭dim is B♭, D♭ and F♭ — 1, b3 and b5 counted from B♭.