Dominant sevenths

A major chord is content to stay put. Add one more note — the seventh — and it starts leaning, pointing at the chord that comes next. That restless version is the dominant seventh, written C7, A7, G7, and so on. It’s the engine of the blues and the signpost of nearly every song’s homecoming.

What the seventh does

Play G7 and let it ring: something in it wants to move. Land on C afterward and you’ll feel the sentence end. The added note rubs against the chord’s own notes just enough to create pull. Songs use that pull on purpose — and the blues enjoys the friction so much it never resolves it.

The five open sevenths

Each one is a small edit to a shape you already own. (Diagrams: columns are strings, low E leftmost; digits are fingers.)

C7 — C major plus the pinky on the G string
  E A D G B e
  x . . . . o
  -----------
1 . . . . 1 .
2 . . 2 . . .
3 . 3 . 4 . .

A7 — A major with the middle of the shape opened up
  E A D G B e
  x o . o . o
  -----------
2 . . 1 . 2 .

G7 — G major with the high note pulled down to fret 1
  E A D G B e
  . . o o o .
  -----------
1 . . . . . 1
2 . 2 . . . .
3 3 . . . . .

E7 — E major with the ring finger lifted off the D string
  E A D G B e
  o . o . o o
  -----------
1 . . . 1 . .
2 . 2 . . . .

D7 — D major with the triangle flipped upside down
  E A D G B e
  x x o . . .
  -----------
1 . . . . 1 .
2 . . . 2 . 3

Notice the pattern: three of the five (A7, E7, G7) got simpler than their major versions — a finger lifted or moved lets the seventh ring on a string that was fretted before. Play each major-then-seventh pair and listen for the same added restlessness every time.

Movable sevenths

Barre the E7 shape and it travels, root on the low E string, exactly like the barres from earlier:

E7-form barre at fret 3 (G7)
  E A D G B e
3 1 1 1 1 1 1
4 . . . 2 . .
5 . 3 . . . .

A7-form barre at fret 3 (C7)
  E A D G B e
3 x 1 1 . 1 1
5 . . . 3 . .

Same naming rule as always: the root under the barre names the chord, the shape makes it a dominant seventh.

Where dominants show up

• The V chord. In any key, the chord built on the fifth degree pulls home hardest, and it’s usually played as a seventh: G7 → C, E7 → Am, A7 → D. When a song feels like it’s winding up to return home, a dominant seventh is usually doing the winding.
• The blues. In a blues, every chord is a dominant seventh, home included. Try the skeleton of a twelve-bar blues in A, one slow strum per bar:

A7 | A7 | A7 | A7
D7 | D7 | A7 | A7
E7 | D7 | A7 | E7

That grid, with these three open shapes, is a genuine blues. You can now play one — which, as far as Earth music goes, is no small credential.