Root on A string
x4565x
C♯dim is the C♯ diminished chord. Its notes are C♯, E and G. It is the same chord as D♭dim. A compact, tense triad. Every interval in this voicing is a chord tone.
x4565x
Numbers show frets from the low E string to the high e string; x means don't play that string.
C♯dim is the vii° chord in D major. That makes it a natural fit in songs written in those keys.
Every C♯, E and G on the first 12 frets. The filled dots are the root, C♯. Build your own shape from any nearby group.
Other C♯ chords
Other diminished chords