What Bbb?

probably a simple explanation, but while learning music theory i keep coming across Bbb, what the hell is that? To state the obvious normally there is a B and a Bb so whats Bbb?

b double flat?? i dno wat dat is but MEH

(meh 4 skold)

Bbb is the same as an A. It occurs in a Cdim7 chord. The dim7 of C is an A, but because in C an A is the sixth note, and the seventh note would be some kind of "B" (B natural (maj7), B flat (m7), etc), it is written as Bbb (B double flat).