A diminished chord stacks minor thirds to create an unstable chord with strong tension. Cdim consists of C, E flat, and G flat, and it has the quality of wanting to move on to the next chord.
The basic idea
It has little of the stability of major or minor, so rather than sounding it alone for long, it is more often used as a passing chord or as the chord before a resolution. A diminished 7th adds another note for an even more distinctive sound.
Its role in writing and performing
Placing one in the middle of a chord progression creates tension that slips into the next chord by a semitone. It also suits moments where you want a cinematic, jazzy, or slightly uneasy color.
How to try it in a DAW
Try placing a diminished chord briefly between two chords of a basic progression so that it connects by a semitone. Avoid stacking it too far into the low range; listening around the mid range makes the sound easier to follow.
How it differs from related terms
Diminished chords also appear among diatonic chords. Like a secondary dominant, it can create a sense of motion, but its character is different.
Rather than holding strong instability for long, using it briefly to highlight the next landing point makes it easy to handle.
