For this reason, I was searching online about DADGAD guitar last weekend. All I really knew about DADGAD guitar was that it was an open tuning and that you often leave the third of the chord. I know enough basic music theory to grasp what leaving out the 3rd means. The 3rd is the note that makes a chord sound major or minor. The notes in a D chord are D, F# and A, with F# being the 3rd. When you leave out the 3rd it gives the chord a droning, power chord quality that suits Celtic music.
What I didn't know was what you replace this note with. I found a page devoted to DADGAD guitar that suggests there is a formula. Michael Eskin says that as a general rule, when in DADGAD he replaces the 3rd of the chord with either the root, the 5th, the 7th or 9th depending on what's easiest to play, what suits the melody, and/or what reinforces the tonal root of the tune.
For the D chord you would replace any F#’s in your typical chord shape with one of the following notes: D, A, C or E. (D is the root note, A is the 5 of the chord, and C or E are the 7 or 9 notes one whole step away on either side of D). It doesn’t really matter if you are doubling up or tripling up the D or the A notes – that helps the root tone of the chord ring out. If you can get by on just the notes D and A for your "D" chord - such as DDDAAD - those will work fine.
A Gmajor chord would typically use the notes G, B and D. Using this same formula, when you need to make a G chord in the key of D you’ll remove the B note and replace it with either G, D or A. Why A? Because this G chord is being used in the role of the IV chord in the key of D, using an A note makes sense because the sound of the A will reinforce the D note to which the tune centers around and resolves to. GDDGAD.
This might be an oversimplification, and there's a good chance I misinterpreted the instructions. At the very least, I think I know know a little bit more than I did before. I would like to hear what others thoughts are on this.