Double drop D tuning: DADGBD, also known simply as double drop D, is an alternative guitar tuning: E strings are tuned down ("dropped") one whole step (2 frets) to D rather than E as in standard tuning (EADGBE).
The main use for double dropped D is so that guitarists can play intervals of a fifth with one finger on the bass strings, and play the treble side of a barre chord. Some recordings that make effective use of this tuning are "Black Water" by The Doobie Brothers, "The Loner," "Cortez the Killer," "Don't Let It Bring You Down," "Ohio" and "Cinnamon Girl" by Neil Young, "Choctaw Bingo" by James McMurtry, "Find the Cost of Freedom" by Crosby, Stills, Nash, and Young, "Going to California" by Led Zeppelin, "The End" by The Doors, "Devils & Dust" by Bruce Springsteen, "Bryter Layter" by Nick Drake and "Nobody's Fault but My Own" by Beck.
Chords in double dropped D tuning are formed as they are in standard tuning, with the exception of the first and sixth strings, which are either omitted or fretted one whole step higher: