Learn to Play in Dropped-D Tuning
One of the things that prevents many guitarists from experimenting in altered tunings is the hassle of retuning. Dropped-D tuning (D A D G B E) lowers the bass string only slightly but opens up a new world of tonalities and textures. To get into dropped-D tuning, lower your sixth string from E down to D. Use your fourth string as a reference point; your sixth string should be exactly an octave lower than your fourth string. You don’t need to go very far—on a steel-string guitar, it typically takes less than a full turn of the tuning peg.
Once you’ve arrived at dropped-D tuning, indulge yourself with the huge D chord you now have at your disposal. It’s fingered exactly the same as a standard-tuning D chord, but you can strum all six strings to get a nice low bass.
Keep in mind that any chords played on the top four or five strings (C, A, Am, Bm, etc.) in standard tuning don’t need any modification at all in dropped D. Any chords that incorporate notes on the sixth string, however, will need to be modified slightly—you have to raise the notes on the sixth string up two frets. For example, the standard tuning Em chord on the left becomes the chord on the right in dropped-D.
Excerpted from Alternate Tunings Guitar Essentials