Root on A string
x23x34
Bdim7 is the B diminished 7 chord. Its notes are B, D, F and A♭. Stacked minor thirds. Its symmetrical sound makes it useful as a passing chord.
x23x34
Numbers show frets from the low E string to the high e string; x means don't play that string.
Every B, D, F and A♭ on the first 12 frets. The filled dots are the root, B. Build your own shape from any nearby group.
Other B chords
Other diminished 7 chords