A Diminished
dim
0 - 3 - 6Root + minor 3rd + diminished 5th
About A Diminished
A diminished is A, C and E♭ — a minor triad with the fifth lowered — and like every diminished chord it exists to move. On guitar it appears mostly as a three-string shape between two stable chords, and it is gone before the ear has time to object.
The notes
A, C, E♭. Two stacked minor thirds, symmetrical under inversion, with a tritone between the outer notes. No note is more the root than any other.
Add an F♯ below and the chord becomes F♯7♭9. That relationship is the reason diminished shapes are so economical in jazz: one grip serves four dominant chords, a minor third apart.
How it is used
As a passing chord between A minor and G, or between two chords a tone apart in either direction. The bass moves by a semitone and the diminished chord covers the step.
It also functions as the vii of B♭ major, though that role is more theoretical than practical — in real music the chord is nearly always a connector.
Guitarists usually voice it on the middle strings and move it in minor thirds, since the symmetry means the same shape three frets up contains the same notes. That property makes the chord quicker to place than to explain.
Where it shows up
Ragtime, early jazz and mid-century popular song, where diminished chords appear as a matter of routine. Later idioms use them sparingly, and their presence now sounds deliberately retrospective.
In minor keys the chord built on the raised seventh degree is diminished, so A diminished belongs to B♭ harmonic minor — one of the few places it appears as harmony rather than as motion.