The G Diminished Arpeggio contains the following notes:

G – Bb – Db

It is essentially an G minor chord, with a flat 5.

Open Position

Here’s how to play the G diminished arpeggio in the open position.

G Diminished Arpeggio - Positions Along the Fretboard
Diminished-Arpeggio-Notes-Key-G-Pos-Open-Shape-0

G Diminished Arpeggio CAGED Positions

Here are the 5 CAGED positions for the G diminished arpeggio on the guitar (notes and tabs).

3rd Position (lowest fret is 3):

Diminished-Arpeggio-Frets-Key-G-Pos-3-Shape-1
Diminished-Arpeggio-Notes-Key-G-Pos-3-Shape-1

5th Position (lowest fret is 5):

Diminished-Arpeggio-Frets-Key-G-Pos-5-Shape-2
Diminished-Arpeggio-Notes-Key-G-Pos-5-Shape-2

8th Position (lowest fret is 8):

Diminished-Arpeggio-Frets-Key-G-Pos-8-Shape-3

Diminished-Arpeggio-Notes-Key-G-Pos-8-Shape-3

9th Position (lowest fret is 9):

Diminished-Arpeggio-Frets-Key-G-Pos-9-Shape-4
Diminished-Arpeggio-Notes-Key-G-Pos-9-Shape-4

12th Position (lowest fret is 12):

Diminished-Arpeggio-Frets-Key-G-Pos-12-Shape-5
Diminished-Arpeggio-Notes-Key-G-Pos-12-Shape-5

Further Reading:

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