When dealing with guitars, an open tuning is where the strings are tuned differently from “standard” tuning (EADGBE) in such a manner that strumming with no strings fingered or fretted generates a major or minor chord. For example, tuning a guitar to DGDGBD low to high is Open G, and strumming with no strings fretted sounds a G major chord in second inversion. Open tunings are often used when playing slide guitar, and were popular with early folk and blues players, and later utilized by rock guitarists with heavy blues influences, such as Keith Richards and the Blues Explosion. Open G was used on the Stones classic “Honky Tonk Women,” among others.
One advantage in using an open tuning is that you can change chords by simply placing a finger across all six strings and moving to the desired fret. Open tunings also make fingerpicking, particularly root-fifth bass patterns very easy to play.
Open G (DGDGBD) and Open D (DADF#AD) are the most popular open tunings, though any tuning that can be achieved without making the open strings too loose or too tight can be used.