G Diminished
dim
0 - 3 - 6Root + minor 3rd + diminished 5th
About G Diminished
G diminished is three notes — G, B♭, D♭ — stacked in equal minor thirds, and like every diminished triad it has no root worth defending. On guitar it is a passing shape, used to move a bass line by a semitone and gone within a beat.
The notes
G, B♭, D♭. The D♭ is the diminished fifth, and the tritone it forms with the root is what destabilises the chord. Two equal intervals mean the shape repeats under inversion.
Add an E below and the chord becomes E7♭9. That relationship is why one diminished grip covers four dominant chords in jazz — a fact that makes the shape far more useful than its own harmony suggests.
How it is used
Between two chords a tone apart, most often between G and Am in the key of C, or as a chromatic step in a descending line. The chord is never a destination.
Guitarists voice it on three strings and move it in minor thirds — because of the symmetry, the same shape at the third, sixth and ninth frets gives the same four notes.
Where it shows up
As the vii of A♭ major in theory, and as a connector everywhere else. Ragtime and early jazz used diminished chords as a matter of course; later idioms use them more sparingly.
It also appears in minor keys as the chord built on the raised seventh degree — in A harmonic minor, G♯ diminished plays exactly this role, and the shape is identical one fret up.
For a guitarist the useful habit is to learn one diminished shape and move it in minor thirds. Four positions cover every diminished chord there is, because after twelve frets the notes repeat exactly.