Modal mixing is the practice of borrowing notes from a parallel key and is most commonly found in major keys, taking intervals from their parallel minors. Modal mixing changes the quality of a chord but does not necessarily alter its function. This musical device can be used in melodies, in one or more chords, along with extended tonicizations, as well as modulations.
Modal mixing often uses the parallel minor of a major and not necessarily the relative minor. For instance, the parallel minor of C Major is C minor. But, the relative minor of C Major is A minor. To find the relative minor of a major scale, simply count up to the sixth degree or note and that’s it. But, since the parallel minor contains the same tonic note, the intervals differ.
Modal mixing is also known as borrowed chords because the application takes elements from a parallel minor. These chords can be outside the tonic key but will still be modally related to the tonic in one way or another. This creates a new sound and makes a musical piece more interesting for the listener.