Mixture is a term used to describe the process where notes, and chords, from a minor key are used in the parallel major key, or the reverse; this is known as simple mixture, (Aldwell & Schachter, Harmony and Voice Leading, 2nd ed). Using chords in this way is also known as Borrowed chords. Because the harmonised melodic minor scale already offers a variety of chords, usually mixture is used from the minor key to the major key. One common exception to this is the use of a major tonic chord in a minor key: known as a tierce de Picardie, (Picardy third).
The most common chords taken from a minor key which are then used in the parallel major are, the diminished chord on ii, the minor chord on iv, and the major chord on the flattened VI degree
As you can see, the use of these chords in the parallel major changes the sounds of all three chords: minor ii becomes a diminished chord, major IV becomes minor, while minor vi becomes major; the sixth degree is also flattened.
Essentially, these types of borrowings are used for expressive purposes but they also provide contrast, especially when the mixture introduces an extended passage in the parallel major or minor key, or in the key of the borrowed chord.
In this way, a passage may be immediately repeated in the parallel key,
or, a borrowed chord is used as a pivot chord to modulate to a new key.
In this example the borrowed minor subdominant from C major’s parallel minor is used as a pivot chord to modulate to the key of F minor.
The borrowed chord does not have to be the tonic of the new key, but may simply be part of the new key.
In the following example, the borrowed minor subdominant in C major becomes chord ii in E flat major and is used as a pivot chord to lead the music into this key.
A further use of this concept is often termed Secondary Mixture. Secondary mixture occurs when a triad is altered and this alteration does not occur through simple mixture; typically this secondary mixture alters the third of the triad, but other notes may also be changed (this creates what is sometimes also termed Altered chords). Probably one of the most common examples of secondary mixture is the use of a major mediant chord in a major key.
Composers began using the major mediant chord to provide more contrast between it and the tonic and dominant chords. Often the altered mediant would become a modulatory destination especially when moving between the tonic and dominant keys.
Other examples of secondary mixture include using a major triad on the sixth degree of a major scale,
or, using either a minor or major triad on the leading note of a major key.
As with simple mixture, the use of altered chords created through secondary mixture is probably more common in major keys, however, it can also be found in a minor key. Alterations in a minor key include, using minor chords on the submediant and flattened supertonic degrees.
A final extension of this concept is termed Double Mixture. Double mixture occurs when secondary mixture is applied to a chord which has resulted from simple mixture.
For example, the major chord on the flattened sixth degree of a major scale can become a minor chord on the flattened sixth degree.
Mixture and W. A. Mozart’s ‘Dissonant’ Quartet Intro