A Dominant 7th
7
0 - 4 - 7 - 10Major + minor 7th
About A Dominant 7th
A7 is the blues chord of the open position. Two fingers, five ringing strings, and a G on the top that turns the bright A major into something that leans. In the key of D it is the dominant; in a blues in A it is the whole song.
The notes
A, C♯, E, G. The G is the flat seventh, and the tritone it forms with C♯ is what gives the chord its edge. In the open shape the G sits on the high E string at the first fret — one of the easiest notes to reach on the instrument.
The usual fingering drops one finger from the A major shape rather than adding one, which is unusual: most sevenths are harder than their triads. A7 is easier, and that has consequences for how often it is played.
Why blues players live on it
A twelve-bar blues in A uses A7, D7 and E7 — all three available in open position, all three with the flat seventh built in. The form never resolves, because every chord is a dominant, and the resulting suspension is the sound of the genre.
The open A7 also leaves the low A ringing, which supports the shuffle rhythm without a bass player. That is why so much solo blues guitar is written in this key.
Where it shows up
As the V7 of D major — the most common single use outside blues. A7 to D is the cadence at the end of countless folk tunes.
It also appears as a secondary dominant in the key of G, where it precedes D and briefly points the ear away from home. That trick, a chord borrowed from a neighbouring key, is one of the first steps beyond three-chord songwriting.