Open G tuning gives a chord of G when all the strings are played open, without any frets being held down. It uses the following notes: D G D G B D . Open G tuning is sometimes called “Spanish Tuning”.
To change a standard tuning (EADGBE) to Spanish or Open G tuning, tune the first, fifth and sixth strings down one whole-step. You can do this either using a regular electronic guitar tuner, or by comparison to the other strings, which are left untouched.
Starting with standard EADGBE tuning,
Tune the 1st string down from E to D (same as 2nd string 3rd fret)
Tune the 5th string down from A to G (same as 6th string 3rd fret)
Tune the 6th string down from E to D (same as 4th string but an octave lower)
When you strum all six strings without fretting any of them, you should hear a pleasant-sounding G chord.
Note that this open G chord now contains three G notes (covering 3 octaves), two G notes (covering two bass octaves) and a single B note ( in the treble octave). The chord is therefore slightly ‘unbalanced’, but is a very useful voicing of the G chord.
Open G tuning has been used by many rock and pop guitarists including Keith Richards of the Rolling Stones – most notably in early versions of Jumping Jack Flash and others.
Give it a go and enjoy experimenting with open tunings.