Great lesson thanks

I personally like E and A because it's possible to tune up my acoustic and play some bottleneck without changing the action; whereas tuning down to G or D does not work as well without adjusting the action on the guitar.

It is also probable that at least Open G was used in the South and called by other names, depending on who was handing it down to who and when. But Open G Spanish is also a standard traditional tuning for the five string banjo; less the sixth string.  In Spanish the sixth string is a D, not a G, and is rarely used.  I have always thought this was a natural transition to use this tuning on guitars in the Delta as soon as affordable guitars were available.  Prior to that folk musicians played mostly mandolins, banjos and fiddles.  People tend to forget that blues players played different styles of music to suit their audience in order to make a living.  Gary Davis played marches, and just about any song he could play in the guitar.  

As far as the G/A and D/E relationships, I think the key concept is that the intervals are the same; which means the chord and scale positions are interchangeable between these tunings. So as long as you are thinking I, IV, V, it's easy to play the same song exactly the same way.  Why have both?  Depending on the instrument and string tension you are comfortable with one tuning might be more comfortable than the other.  I know a lot of Delta players like Big Joe Williams played in Spanish capoed up to A, probably because his strings were too heavy to comfortably tune up to open A.  

Corey:  I think these lessons are terrific.  I think it's very important to identify really key blues techniques that reoccur in addition to learning songs.  They form sort of a tool box.  I thought I would offer a little clarification on the origin of the terms "Spanish" and "Vestapol", for those who are interested.  They did not, actually, originate in the South.  They originated during the parlor guitar craze, which was an attempt to market rather expensive parlor guitars to well-to-do families in the North East.  In order to sell the guitars, they had also to provide music which was easy to learn.  Spanish tuning gets it's name from what was an enormously popular song at the time called the "Spanish Fandango" and it was published in the open G tuning we call Spanish.  It became associated with the song. It was not a blues song, but virtually all blues players played it because they were trying to put food on the table and they played popular songs when requested.  Vestapol is actually the name of another very popular song of the period, published in Open D tuning; and the tuning became associated with that song as well.  And blues players almost all played that song too.  The two songs were pretty good primers on finger picking in open tunings, but they were not slide songs.  

Thanks Corey, I'm finding these Fundamentals series extremely helpful

I'm really enjoying thanks this for taking the time in doing this, thanks