The B diminished seventh chord is a 4-note chord consisting of the notes B, D, F and Ab.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Bdim7.
Chords that are a subset of Bdim7. These contain less notes but all of them are included in Bdim7.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the Bdim7 chord. Feel free to save or share the image as needed.