Four & Twenty Tuning (D.A.D.D.A.D)

This tuning was popularized by Crosby, Stills, Nash and Young in their songs Four and Twenty and Suite: Judy Blue Eyes (which was transposed up one whole step). Strumming the open strings of the Four and Twenty tuning sounds a chord that is harmonically ambiguous - neither major nor minor, and perhaps this accounts for some of the charm of these songs. The doubled string adds an interesting flavor to finger picked passages, since the same tones repeat, though with slight timbral differences due to string thickness and weight.

Since the tuning has multiple D's and A's, there tend to be numerous variants possible on any given chord form. For instance, an open position D minor 7 can be played (along with many other variants) while a B minor 7 could be fingered. The general rule is that whenever a given fret is played on (say) a D string, that same fret can be played on any other D string.