How to Play an A,D7 to A7 Guitar Chord Transition

"I'm Rick Tobey, on behalf of Expert Village, and this is Blues Progressions Part Two. So now we're going to take those three songs, three chords, open chords, and we're going to put them together and play a twelve bar Blues progression in A. Now each bar is a measure; a measure has, in this case, four beats. So each group of four beats is called a bar and I'm going to play this progression and I'll count each bar so you'll see how the twelve bar progression sounds. One, two, three, four. Two. Three. Four. Five. Six. Seven. Eight. Nine. Ten. Eleven. Twelve. So that's a twelve bar blues in A."