You may be familiar with the term mode used to describe the "Church modes" (Ionian, Dorian, etc.). In common-practice music, however, the term is used for major and minor: the major mode and the minor mode.

Mode mixture (also known as modal borrowing) is a technique that involves using notes or chords from one mode in another mode. Regardless of whether major borrows from minor or vice versa, the borrowed chords retain their usual harmonic functions (or lack thereof) since they are standing in for their diatonic counterparts.

In most cases, composers borrow from the minor mode when in a major key. For example, using lowered scale degrees 3, 6, and 7 (me, le, and te) in isolation in a major key serves as a reference to the minor mode. Similarly, composers might use any of the diatonic minor chords (i, ii°6, III, iv, VI, vii°7, etc.) when in a major key. Artists in popular music occasionally use the minor iv chord in a IV-iv-I progression to increase tension via the chromatic line la-le-sol. (In the example below, the IV and iv chords are written without parentheses. In popular music, IV has different functions depending on the context and, arguably, the musician.)

Mode mixture in a minor key is less common overall, but there are two chords worth mentioning here. One is the major IV chord, usually in the context of something like a IV6-V6/5-I progression, which features a rising la-ti-do bass line. If iv6 to V6 was used instead, it would involve a le-ti bass line, resulting in an augmented second error. The other chord is the major I chord, which typically appears at a prominent cadence or at the very end of a minor-mode piece. The term Picardy third refers to the raised scale degree 3 (mi) over the major I chord in these contexts.