Watch the Extra Five online guitar lesson by David Hamburger from 30 Beginner Fingerstyle Blues Licks You MUST Know

Officially, chord substitutions like the B7 in the second half of measure two are called "secondary dominants". I first discovered they could work on the blues while checking out pianist Ray Bryant's solo recordings, but subsequently realized that these kind of "extra V chords" are all over early jazz versions of the twelve-bar blues.