modal interchange
The technique of borrowing chords from a parallel mode to enrich the harmonic palette
In Depth
Modal interchange, also called mode mixture, allows a composer working in a major key to use chords from the parallel minor (or vice versa), or to borrow from any parallel mode. The most common example is using a minor iv chord in a major key context, which produces a bittersweet quality. Other popular borrowings include ♭VII, ♭VI, and ♭III from the parallel Aeolian mode. The technique is pervasive in pop, rock, and film music, where it adds emotional depth without full modulation.
The Beatles used modal interchange extensively — the ♭VII chord in songs like Let It Be comes from the parallel Mixolydian mode.