C Diminished (Cdim)
1+b3+b5
Notes in C Diminished
C (root)EbGb
Inversions
Same notes as C Diminished, with a different chord tone as the lowest note. Useful for smoother movement between chords instead of jumping around the keyboard.
Root position
C in the bass
1st inversion
Eb in the bass (Cdim/Eb)
2nd inversion
Gb in the bass (Cdim/Gb)
The notes C, Eb and Gb make up C Diminished.
Stack two minor thirds and the fifth shrinks along with the third, producing a triad that never feels at rest. Diminished chords are unstable by design, built to lean somewhere else, usually resolving up by a half step. Played alone it can sound almost comical, but tucked between two other chords it's one of the most useful tension devices in tonal music.