E Dominant 7th
7
0 - 4 - 7 - 10Major + minor 7th
About E Dominant 7th
E7 is the loudest dominant on the guitar. It keeps all six strings of E major and adds a D on the D string — or removes a finger, depending on which shape you learn — and the result is the chord that opens more blues songs than any other.
The notes
E, G♯, B, D. The D is the flat seventh, and there are two common ways to get it: lift the third finger from the E major shape, leaving the D string open, or add the little finger at the third fret of the B string for a fuller version.
The first is easier and thinner, the second is heavier and puts the seventh on top where it is more audible. Blues players use both, usually in the same song.
Why E is the blues key on guitar
The open low E is the lowest note the instrument has, and in a blues in E it is the tonic. Riffs can be played against it without fretting the bass at all, which is why so much of the repertoire is written here rather than in a key that suits the voice better.
The other two chords of the form, A7 and B7, are also open shapes. Three chords, no barres, and the deepest note available as the root — that combination exists in no other key.
Where it shows up
As the V7 of A major, and as the tonic of a blues in E. It also serves as a secondary dominant in the key of D, pointing briefly at A before the song returns.
In jazz it is more often played as a barre with the root on the A string, because the open version cannot be altered easily — and altering the dominant is most of what jazz harmony does with it.