The chord symbol for a diminished chord is B or dim. The full name of this chord is diminished seventh but it is usually referred to as a diminished chord. Here is the most common chord shape for the diminished chord.
EbB/AB CB/F#B
This diminished chord shape has four different names. The diminished chord can take its name from any note in the chord, e.g., the shape shown here contains the notes E flat (Eb), C, A and F sharp (F#). So this shape can be either Ebdim, Cdim (CB), Adim (AB) or F#dim. If this shape was moved up one fret it would be Edim (EB) or C#dim, Bbdim or Gdim etc. To locate any diminished chord simply play this diminished shape so that it includes the letter name of the chord.