Mixture is the use of chords from a parallel key (i.e., parallel major, minor, or mode) in a piece of music.
Chords from minor that are most often included in a major key include:
iio iv bVI
Other chords that are borrowed from minor and put into major (especially in jazz compositions) include:
bIII and bVII
A particular use of the bVII chord in jazz is called the Backdoor progression:
iv – bVII – I
The chords from major keys most used in minor keys are the major V – the dominant, and the viio – the leading tone triad.
A chord from major that is used in minor keys is the major I chord – usually used at the end of a piece. This use of the major I is called the Picardy Third.