The C Major 9th 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.
Cmaj9 is C, E, G, B, D — the 1, 3, 5, 7 and 9 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, B, D 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.