Open tunings are widely used in contemporary, fingerpicking guitar. Ry Cooder, Leo Kottke and John Fahey are some popular guitar players who frequently use open tunings.
An open tuning occurs when the open strings on the guitar are tuned to from a chord.
For example, the notes which make up the G Major chord are G, Band D. If we lower the first, fifth and sixth strings one tone, (2 frets), the notes of all the open strings now are D, G, D, G, B, D. We now have a G Major chord on the open strings, this is called open G tuning.
Open tunings give the guitar a different sound and tone depending on what tuning is used, because you can play chords you cannot normally position in standard tuning. The following pieces show four popular open tunings but there is an endless amount of different combinations the open strings can be tuned to. Only the tablature is shown because of the difficulty in reading open tuning guitar music.