A B Diminished (Bdim) chord contains the notes B, D, F. It is built from the diminished formula 1 - ♭3 - ♭5, with B as the root.
What is a diminished chord used for?
Tension, not rest. A diminished chord stacks two minor thirds, producing an unstable sound that pulls hard toward resolution — so it's typically used as a short passing chord between two stable chords rather than somewhere a progression settles.