About this tuning:
This is a D suspended fourth tuning (Dsus4).
ie. when all the strings are played open, the sound is of a suspended fourth chord (Dsus4).
Some of you may be saying, whats a suspended fourth chord?
Well, all chords have a configuration, a formula you might say! The formula for a suspended fourth chord is the 1st, 4th and 5th notes of a major scale. If we look at the scale of D major these notes are D, G, A (more on chord formulas later).
It is very commonly referred to as a Modal tuning although to be quite honest I'm not entirely sure why or where this phrase originated.