Watch the Slidin' 6ths online guitar lesson by Jeff McErlain from Essentials: Solo Electric Blues
A little theory, in music the term interval means the distance between two notes. In this blues we are using the interval of a 6th quite a bit. What that means is the two notes we are playing are 6 notes apart from each other within the key. That's important to understand as we are changing keys ever so slightly here for each of the three chords. The riff uses scale tones (notes from the scale) to embellish each chord. The first chord is E7 and the corresponding scale for that chord is E mixolydian, EF#GABC#D. Our first use of the 6th is between the B and G# notes which is contained in the E7 chord itself. If you count using B as the first note, we can see that G# is six notes above in the key, giving us a 6th of course. For the riff we move that interval up to the next notes in the scale, C# and A. Our next set of notes is the D and B. As you can see, all of these intervals are 6ths within the key. The A7 chord uses an A mixolydian scale, ABC#DEF#G and the B7 take a B mixolydian scale BC#D#EF#G#A. To properly outline each chord we must stick to the notes of the proper scale. Once you get this idea down it really opens up doors.