The C♯ Diminished chord and everything around it — the notes and how they are built, the shape on piano and guitar, the sound, and the keys it belongs to.
C♯dim is C#, E, G — the 1, b3 and b5 above the root C♯.
Root notes are red. There is no standard open shape for this chord — any group of these notes on the neck sounds the chord.
Every position of the chord tones on the ukulele neck — any group that sounds only C#, E, G plays the chord.
Work out the chords of a whole key with the Scale & Chord Finder, name a chord from its notes with the Chord Identifier, or browse every chord tool. The theory behind how chords are built is in the free Cloud Atelier music guides.