Drop D tuning, also known as drop tuning or slack key, is an alternative way to turn a guitar. To tune your guitar to Drop D you must:
  1. Tune your guitar to Standard Tuning.
  2. Tune DOWN your E strings (stings 1 and 6) to D.* You can use your D string (sting 4) to do this.

Drop D is an interesting way to get more diversity out of your guitar. Smashing Pumpkins, Foo Fighters, and Days of the New# all use Drop D tuning in some of their best songs.

*Tuning string 1 (your thinnest E string) down is optional.
#Days of the New use Drop Db tuning.