F Diminished (Fdim)
1+b3+b5
Notes in F Diminished
F (root)AbCb
Inversions
Same notes as F Diminished, with a different chord tone as the lowest note. Useful for smoother movement between chords instead of jumping around the keyboard.
Root position
F in the bass
1st inversion
Ab in the bass (Fdim/Ab)
2nd inversion
Cb in the bass (Fdim/Cb)
The notes F, Ab and Cb make up F 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.