Ray wrote
So I turned around and went home and read the intro to the exercises. DADGAD!
I've been reading about it but never had any encounter with it til now. All cool and groovy again. Ag you know, I was embarrassed for a while but then I thought it was quite a hoot.
I've been vowing to give DADGAD a try - but somehow I never find the time.
Jimmy Page used DADGAD on some Led Zeppelin numbers (I can't say off-hand which ones, and that wasn't the only alternate tuning he used with LZ).
"Invention" or "discovery" of DADGAD is usually attributed to the late Davy Graham. Graham himself said that what he did was take a tuning commonly used on the oud and move it to the guitar.
If you really want to get into unusual tunings, then try some Nick Drake pieces. Drake used a lot of different tunings. Though I think it more the case, given that Drake didn't write much down in the way of tabs, that serious Drake scholars have had to figure out what tunings he used, or find tunings that made various things possible. There's no doubt that he did use a lot of different tunings (and not the more common ones like DADGAD or drop-D) because on the rare occasions when he played live there was a lot of retuning going on between each number. Joe Boyd (who should know) also makes references in his book to Drake's use of unusual tuning and his retuning between demonstrating or recording each number.
Returning to Led Zeppelin... I'm fairly sure that "Black Mountainside" and "Rain Song" are in DADGAD.