C Diminished
dim
0 - 3 - 6Root + minor 3rd + diminished 5th
About C Diminished
C diminished is a chord built entirely from minor thirds — C, E♭, G♭ — and it belongs to no key comfortably. Its symmetry is the point: stack another third and you get a diminished seventh, which divides the octave into four equal parts and can resolve almost anywhere.
The notes
C, E♭, G♭. Both intervals are minor thirds, which makes the chord symmetrical — invert it and the shape repeats. That is why it has no strong root: any of its notes can act as one.
The G♭ is the flat fifth, and it forms a tritone with the root. Unlike a dominant seventh, where the tritone points somewhere specific, here it simply destabilises.
How it is used
Almost always as a passing chord. A diminished triad between two stable chords smooths the movement of a bass line by a semitone, and the ear accepts it as motion rather than as harmony.
On guitar the shape is usually played on three or four strings only, because the full symmetrical voicing is awkward and the chord rarely needs volume — it is a hinge, not a destination.
Where it shows up
As the vii of D♭ major in strict theory, and as a chromatic connector everywhere else. In jazz the diminished seventh version appears between two chords a whole tone apart, filling the gap.
It is also the sound of silent-film melodrama and of the moment before a resolution in early jazz — a cliché precisely because it works so reliably.