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