piano chord
Cdim7
C1E♭♭3G♭♭5B♭♭♭♭7
Three minor thirds stacked. The seventh is doubly diminished, which is why it is written with a double flat.
What is true of it
Cdim7 is C, E♭, G♭ and B♭♭ — 1, b3, b5 and bb7 counted from C.
B♭♭ is a double flat, and it has to be: every letter from C upwards is used once, so the bb7 cannot be written any other way.