A Diminished (Adim)
1+b3+b5
Notes in A Diminished
A (root)CEb
Inversions
Same notes as A Diminished, with a different chord tone as the lowest note. Useful for smoother movement between chords instead of jumping around the keyboard.
Root position
A in the bass
1st inversion
C in the bass (Adim/C)
2nd inversion
Eb in the bass (Adim/Eb)
A Diminished is built from A, C and Eb.
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.