Root on A string
x3454x
Cdim is the C diminished chord. Its notes are C, E♭ and G♭. A compact, tense triad. Every interval in this voicing is a chord tone.
x3454x
Numbers show frets from the low E string to the high e string; x means don't play that string.
Cdim 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