To add interest and expressivity to their music, composers oftentimes mix elements of major and minor freely. Mode mixture capitalizes on the close tonal relationship between a major key and it's parallel minor key and vice versa.
Consider the following example:
Schubert: Die liebe Farbe
The harmonic language of the piano introduction is unequivocally b-minor. As the voice enters, the musical texture thins considerably. The melodic line sustains the feeling of b-minor in the first phrase, but the appearance of d# in the second phrase (as the persona recalls his beloved) poetically invokes B major. Note that d# does nothing to alter the triads harmonic function--it is still heard as a tonic, only the major version.
The invocation is shortlived, however, for the same text is repeated and led to a PAC in b minor. Schubert, a master of setting music to text, allows the pianist to engage in the musical drama. Note how d# turns up in the bass as part of I6and then sinks to d-natural with the arrival of the cadential 64 .
The d# / d-natural conflict is a product of simple mixture, that is, scale degree 3 from B major is incorporated in a song that begins and ends in B minor.
Borrowing scale degree 3 or 6 or both 3 and 6 from the parallel key is called simple mixture. It changes the quality of some but not all diatonic triads and seventh chords.
Altering the 3rd of of diatonic triads other than I/i, ii0, IV/iv, and vi/VI, where such a such an alteration does not result from simple mixture, is known as secondary mixture.
For instance, the minor dominant (v) or the major mediant (III) do not result from simple mixture.
Examples for Analysis
Listen to the Introduction to Haydn's Symphony No. 100 and mark each formal cadence. Even though the harmonic language of the opening is clearly G Major, some aspects of G minor are also in evidence. See if you can locate in the score accidentals that convey the parallel minor mode.
Haydn: Military Symphony
Assignment: Copy out the First violin and Cello parts, simplifying the rhythm where appropriate. Supply figured-bass for your two-voice setting--make sure accidentals resulting from mode mixture are correctly notated in the figures.