Mixture, Mischung. Mixture refers to the borrowing of notes or chords from the parallel key. In B$ major, the B$ minor chord is understood as mixture, as is E$ minor. Both are borrowed from the key of B$ minor. Secondary mixture involves the employment of other altered chords based on the roots of the original diatonic scale. For example, in B$ major, D$ major is simple mixture, III of B$ minor, but D major is secondary mixture, an alteration of the quality of the diatonic chord, D minor. Much less frequently encountered, double mixture is secondary mixture applied to simple mixture. In B$ major, D$ minor would be an example of double mixture.
Simple mixture occurs in Variation 2, and is the basis of Variations 5, 6, and 13. Secondary mixture is used in Variation 9. As shown below, simple mixture provides a direct opposition of major and minor sonorities in Variation 10.