Watch the Clear for Approach online guitar lesson by Brad Carlton from Chord Studies: Slow Blues Progressions Vol. 1
This example is a twelve bar blues in the key of A. At the end of the progression in the twelfth bar, you have what's known as an "approach chord", in that you are moving down the same form from the F7 one half step to land on the E7. Be on the lookout for guide fingers and pivot fingers to give your fretting hand more economy of motion as you play through the changes.