Watch the Alternate Tunings online guitar lesson by Rich Maloof from Guitar 201: Breakthroughs
Dropped D (or drop D) tuning is based on a pretty simple concept: The lowest string is tuned down a step from E to D. The tuning allows a lower bottom end and puts some bass notes within reach that would otherwise be a stretch. But be careful! Any chord shapes from standard tuning that incorporate a 6th-string note will now require you to play that bass note up one step (two frets). Look at the chart to see how dropped D tuning changes some shapes you thought you knew.