Topic summary
Open tuning

Extracted from the Wikipedia article Guitar tunings.
Open tunings
Open tunings may be chordal or modal. In chordal open tunings, the open chord consists of at least three different pitch classes. In a given key, these are the root note, its 3rd and its 5th, and may include all the strings or a subset. The tuning is named for the base chord when played open, typically a major chord, and all similar chords in the chromatic scale are played by barring all strings across a single fret. Open tunings are common in blues and folk music. These tunings are frequently used in the playing of slide and lap-slide ("Hawaiian") guitars, and Hawaiian slack key music. A musician who is well known for using open tuning in his music is Ry Cooder, who uses open tunings when playing the slide guitar.