piano chord
Fdim7
F1A♭♭3C♭♭5E♭♭♭♭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
Fdim7 is F, A♭, C♭ and E♭♭ — 1, b3, b5 and bb7 counted from F.
E♭♭ is a double flat, and it has to be: every letter from F upwards is used once, so the bb7 cannot be written any other way.