E7, A7 and D7: the chords of a blues in A
The three open dominant 7 shapes that make up a 12-bar blues in A, and what each one is doing in the form.
A blues in A uses three chords: A7 as the I, D7 as the IV and E7 as the V. All three are dominant 7 chords, which is unusual — in most other music only the V chord would be a 7th. That wall-to-wall dominant sound is a large part of what makes a blues sound like a blues.
All three have comfortable open shapes, which is why A is such a common key for blues on guitar. Learn them as a set rather than individually, because you will almost always play them together.
Each carries a ♭7, which is what stops any of them sounding like a place to rest, and that permanent lean is the engine of the form.
- E7 — dominant 7, open shape.
- A7 — dominant 7, open shape.
- D7 — dominant 7, open shape.
Where they sit on the neck
All three have open grips inside the first three frets: E7 is 020100, A7 is x02020 and D7 is xx0212. Between them they leave eight strings ringing open, which is most of why this key sounds so much bigger on a guitar than the same blues in B♭.
They also have movable versions, and those land conveniently close together. The E-shape 7th, with its root on the low E string, puts A7 at the 5th fret. The A-shape 7th, root on the A string, puts D7 at the 5th as well and E7 at the 7th. A whole 12-bar in A can be comped between the 5th and the 7th frets — which is where you were already soloing, so the hand never has to leave home.
What each one is doing
A7 is the I, D7 is the IV, E7 is the V. Every one of them carries a ♭7: G in the A7, C in the D7, D in the E7. A ♭7 leans, so none of the three is a resting place and the form does not settle until it stops.
The 3rds are the other half of the story: C♯ in the A7, F♯ in the D7, G♯ in the E7. None of those three notes lives in A minor pentatonic, so each one exists only while its own chord is sounding. Aim at them and the listener hears the change; ignore them and twelve bars of soloing will sound the same over all three.
One detail worth noticing. The ♭7 of the E7 is a D, which is the root of the D7 — and in the grid above the E7 in bar 9 is followed by the D7 in bar 10. The chord is already pointing at where the form goes next, which is what a dominant chord is for.
The common mistake
Strumming all six strings. The A7 grip has no low E in it and the D7 has neither the low E nor the A, so both need the unused strings damped — the side of the fretting hand's first finger will do it without any extra effort. An A7 with a ringing low E is merely the chord over its own 5th, which is survivable. A D7 with a ringing low E puts an E in the bass and stops being a D7 at all, and under any gain it will sound like the wrong chord rather than a slightly untidy one.