Watch the Open D Tuning online guitar lesson by Andy Aledort from Slide Guitar Power
Open D tuning is identical to open E tuning except all of the strings are tuned down one whole step (low to high: D A D F# A D). In other words, the fifth and fourth strings are tuned normally while the sixth, second and first strings are tuned down one whole step; the third string is tuned down one half step. All of the same scale patterns and chord forms apply to open D tuning as well, as demonstrated here. In open D, barring across all of the strings at the third fret will sound an F chord, barring across all of the strings at the fifth fret will sound a G chord, barring across all of the strings at the seventh fret will sound an A chord, etc.