Modal mixture

So, as we’ve seen, in writing tonal harmony, we can:

• Use the diatonic chords in our key, or else
• Modulate fully to other keys and use those chords, but we can also
• Borrow chords from other keys

This borrowing entails a basic form of ‘chromatic harmony’ which, put simply, encompasses anything that’s not in the prevailing key. We have met a form of ‘borrowing’ already in the chapter about ‘secondary’ or ‘applied’ dominants where we ‘borrowed’ a dominant function chord from another key to tonicize a chord that’s is in our main key. ‘Modal mixture’ is a different form of chromatic borrowing in which we borrow chords from the ‘parallel’ major/minor: so between C major and c minor, or between F major and f minor, for instance.

Which chords can we use?

Let’s take a look at our options by viewing the chords for a major and its parallel (harmonic) minor side by side:

As you can see, almost all of the triads and (even more of the) sevenths are different in the major and minor versions. The exceptions are not subject to modal mixture because they’re the same in both the major and its parallel minor key. Notice that they’re all dominant function chords: V, V7, and viio. This is handy if you want to switch between major and parallel minor — as the dominant function chords are the same, you can just alternate freely, for instance with I, V, i, V, I.

Different degree and / or different quality

There are two kinds of difference to consider here:

1. The quality of the triad (diminished, minor, major, augmented). In comparing harmonic minor with the parallel major, every corresponding chord has a different quality (I – I, ii – iio, …) with the exception of V, V7, and viio as discussed.
2. Some (but not all) chords also involve chromatic alteration to the root. In harmonic minor, this accounts for chords build on scale degrees 3 and 6 (e.g. scale degrees 3 is E in C major, but Eb in c minor).

To notate these chords in Roman numerals,

• if the root pitch is chromatically altered (scale degrees 3 and 6) we need to change the root chromatically e.g. for borrowing the VI chord from minor into major, notate it as bVI.
• if the root pitch is the same, we need no such chromatic inflection, and need only make changes to reflect the quality: by changing between upper / lower case for major / minor and adding or removing the symbols for diminished or augmented.

Melodic minor?!

As always, melodic minor makes life a little more interesting. Because we can have chords built on the raised or lowered 6th and 7th degrees, we have a lot of options. For this reason, the use of an a minor chord in key context of c minor, for instance, is not generally considered to be a case of modal mixture because it is diatonic to the melodic minor.

Example

Here is an example of modal mixture at the start of a Schubert song: ‘Rast’ from the Winterreise cycle. This song begins with mixture of both i and I, and iv and IV. That is, we get major and minor triads on both C and F. Notice how C (major) is also used as an applied chord to tonicize F.