A7 takes the stable major triad and gives it somewhere to go. The new voice is ; together the notes are A, C#, E and G.
sits a whole step below the next root. In the pair below, that single arrival changes a major chord from a resting point into a dominant.
One repeated root moves down a whole step to the flat seventh. The other notes stay the same.
Follow that pull to , the major chord a fourth higher. The third rises while the flat seventh falls, closing around the new root.
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.
Contains A7 and adds ninth, eleventh and thirteenth.
Adds ninth, sharp eleventh and thirteenth, for a dominant sharp-eleventh color.
Contains A7 and adds flat ninth, sharp ninth, sharp eleventh and thirteenth.
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.