General stuff: “Dad-Gad” (DADGAD) is a popular alternate tuning used in Celtic, New Age, pop, rock, and folk styles. DADGAD is the most well-known modal tuning and can be seen as an open Dsus4 (suspended 4th) chord. The resultant open, unresolved sound is a big part of its charm. DADGAD is particularly well-suited for playing modal melodies with droning open strings.

Historic stuff: DADGAD reached the mainstream through a circuitous route. Celtic folk singers and UK guitarists like John Renbourn and Bert Jansch caught the ear of future rock god Jimmy Page, who began using the modal tuning in a rock setting—first, with the Yardbirds, and later, in Led Zeppelin.

Since the sixties, DADGAD has flourished across genres and has found a particular niche within the New Age movement, where it is especially prized for its otherworldly modal quality. DADGAD is the sound of the Yardbirds’ “White Summer,” Led Zeppelin’s “Black Mountainside” and “Kashmir,” Michael Hedges’ “Ragamuffin,” Phil Keaggy’s “Country Down,” Adrian Legg’s “Coging’s Glory,” Bert Jansch’s “St. Fiacre,” and many pieces by Pierre Bensusan.

Technical stuff: DADGAD is accomplished by lowering the sixth, second, and first strings in standard tuning a whole step, to D, A, and D, respectively.

Playing stuff: This phrase conveys the unusual, vaguely ethnic quality associated with DADGAD tuning. There are allusions to Celtic and Eastern modal sounds throughout the folksy figure, and the parallel octave passage, with droning open strings at the phrase ending, is absolutely definitive.
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The rock and pop side of DADGAD is presented in this chordal figure. Ringing open strings are a prominent part of every chord in the phrase, which is the essence of the drone. Note the uncommon fingerings of typical suspended chords and triads, as well as the D5 power chord.
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Stuff! Good Guitar Players Should Know