Chord sequence
A7 D7 A7 D7 A7 E7 D7 A7 B♭7
In every key
Four-note rootless voicings in their two forms, with the 3rd or the 7th on the bottom, played over the root.
Heard in A major
Over A7 · V7/IV
Mode 5 of Major
Over D7 · IV7
Mode 4 of Melodic Minor
Over A7 · I7
Over E7 · V7
Over Bb7 · subV7