D Diminished (Ddim)
1+b3+b5
Notes in D Diminished
D (root)FAb
Inversions
Same notes as D Diminished, with a different chord tone as the lowest note. Useful for smoother movement between chords instead of jumping around the keyboard.
Root position
D in the bass
1st inversion
F in the bass (Ddim/F)
2nd inversion
Ab in the bass (Ddim/Ab)
Play D, F and Ab together and you get D 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.