Harmony
The dominant 7 and its resolution
Why E7 wants to fall to Am — the tritone and the half-step pulls that drive the whole style.
A dominant 7 chord is root–major third–fifth–flat seventh (1–3–5–♭7). In the key of A minor that's E7 = E, G♯, B, D.
Hidden inside it is a tritone — the interval G♯–D — the most unstable sound in tonal music. The ear leans hard toward resolving it.
Resolution: the third G♯ climbs a half step up to A, and the seventh D falls a half step to C. The whole chord collapses onto its tonic a fifth below. E7 → Am.
That single pull is the engine of gypsy jazz. Almost every phrase ending, turnaround and A-section cadence is a dominant falling home.