Watch the Example 1 online guitar lesson by Brad Carlton from Guitar Lab: Boogie-Woogie

This is a three voice passage. The 1st voice is a three voice passage. The 1st voice moves b7, 1,2,1. The 2nd voice moves 5, 6, b7, 6 and the 3rd voice pedals the root. You could view this example as thirds in A mixolydian over an A bass note. Because no 3rd is ever played (either 3 or b3) in this example, you could also view it as thirds in A dorian over an A bass note.