A7 is A, C#, E and G — with the flat seventh added on top.
It sounds restless and pointed at something. The V of a key, and the whole vocabulary of the blues, where even the tonic is one of these.
Dominant — it points a fifth down.
Play it instead of a maj7 or a m7 on the same root, you are comping or blowing over it and want more to play on.
Also written Adom7 or A dominant 7.
A7 is the V of D major. Its third and seventh are a tritone apart, the most unstable interval there is, and each resolves by a half step into the I. The dominant a tritone away shares those two notes, so it resolves the same way.
Listen for: The dominant’s seventh falls a half step to the third of the I. Under the tritone substitute it is the same note.
In a blues, A7 is the home chord rather than a V: it moves to D7 and back without resolving. Its third and seventh each slide down a half step to become the seventh and third of D7, so the shape stays the same, one fret lower.
Listen for: Both notes above the bass drop one fret into the IV and come back: the third becomes the IV’s seventh.
In the turnaround in C major, jazz players usually replace the vi chord with A7, and usually alter it.
Replace the VI7, II7 and V7 of a turnaround with their tritone substitutes and you get Abmaj7, B7, E7, A7. A7 is the last, a half step above the I.
In which keys. Every note of A7 belongs to D major and B minor.
Every note of A7 is in A mixolydian, A lydian dominant and A half-whole diminished.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where A7 sits in it. Every scale.
Change one note of A7 and you have (drop the flat seventh), (the flat seventh becomes the major seventh), (the major third becomes the minor third), (the flat seventh becomes the ninth), (the flat seventh becomes the sixth) or (the major third becomes the fourth).
A7 is a dominant chord — root, major third, fifth, minor seventh. Dominant seventh chords on every root.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.