Watch the DADGAD online guitar lesson by Vicki Genfan from Open Tuning Handbook: Rhythm
Also called Dsus4, this tuning has a very rich history. You will find it often used in Celtic, American roots, blues, and fingerstyle music, and I've used it in my own blend of pop-folk-funk. We can trace some of its roots back to the music of Morocco, in the unique tunings used by the slaves of African descent on their banjos, as well as in the music of the Middle Eastern stringed instrument, oud. According to some, Davey Graham, a very well known and influential guitarist in the British folk music scene of the 60's brought this tuning into the mainstream, and he in turn influenced players as diverse as Bert Jansch, Wizz Jones, John Renbourn, John Martyn, Paul Simon, Jimmy Page, and the French-Algerian guitarist, Pierre Bensusan, who uses DADGAD as his only tuning. It's got a very "open" and suspended sound due to the fact there is no 3rd in the chord. Due to the popularity of this tuning, some guitarists are using the partial capo (called "DADGAD" capo by D'Addario and "Short Cut" capo by Kyser) which allows you to press down only 3 strings (5, 4 & 3) on the 2nd fret of a Standard tuned guitar, and you'll get this same chord, but with the root on E instead of D (EBEABE).
In the Level 1 exercise for this tuning, we'll play a very simple and beautiful finger picking pattern over our ii-V-I-vi progression. In Level 2, we'll use the guitar body to give us some percussion patterns, and in Level 3, we'll add a simple melody using octaves on the 4th & 1st strings.