Watch the Straight Rock Blues 2 online guitar lesson by Jeff McErlain from Blues Survival Guide: Rhythm Edition
Open G tuning or "Spanish" tuning has been around for a very long time. It is most common with slide players. The other common open tuning is open D or "Vestapol" tuning. Most of the time when some one is playing slide they are in an open tuning. There are exceptions but that is generally the rule. Famous blues tunes in open G are "Can't Be Satisfied", "Feel Like Going Home", and "Rollin' and Tumblin" by Muddy Waters. "Little Red Rooster" by Howlin Wolf, "Crossroads" and "Stones In My Passway" are just two in G by Robert Johnson.