Open G tuning is a tuning for guitar, such that the open strings play a G major chord with no fingering or capo.
The standard Open G tuning is D-G-D-G-B-D. Note that this leaves the D as the bottom of the chord, creating an inversion. To rectify this problem, some guitar players will remove the low D string from the guitar and only play with five strings. Dobros in particular use a full six-string tuning with a bottom G, G-B-D-G-B-D. The top two strings are, accordingly, tuned three semitones higher for the bottom string and two semitones higher for the second-lowest string than a standard tuning. As such, some standard guitar strings may require lighter gauges to play the dobro tuning (though not all, since the thicker gauges on the low strings tend to be sturdier and more resistant to tightening than the thinner, more fragile high strings).
The Russian seven-string guitar uses an open G for its standard tuning; it progresses (low-high) D-G-B-D-G-B-D.
Keith Richards is famous for innovating this tuning for rock music on songs such as "Brown Sugar", "Honkey Tonk Woman", and "Start Me Up".
