Experimenting with Open-D Tuning
Open-D tuning (D A D F# A D) is also very popular among singers, slide players, and instrumentalists. Well-known examples include Joni Mitchell's "Big Yellow Taxi," Richie Havens' "Handsome Johnny," and Ry Cooder's "Vigilante Man." The open strings of this tuning explicitly state a major key (D major), but with a little effort it can be used to play in other keys. Slide and blues players often shift this tuning up a whole step to sound in E (E B E G# B E), which increases the tension on the strings and produces a brighter tone. To get into D tuning, you must lower four strings from standard. Tune your sixth, second, and first strings down a whole step and your third string down a half step. Example 13 shows a version of each chord in the key of D. Experiment with these, playing them in different sequences, such as I-IV-V-I, I-ii-V-I, or iii-IV-ii-V, and make up progressions of your own.
Excerpted from Explore Alternate Tunings