What is a modal mixture? It's very common when you are in a major key to borrow aspects from other keys as you go along. The most common way is to borrow chords from the minor key with the same name (C major/C minor). These are called borrow chords, and they are a pretty neat way to spice up your harmony. Because the major and minor key are so close in some respects, you can interchange these chords for some cool sounds. Look at some examples and see what chords you can borrow.
Borrowed Chords from Major
Here, you are in a minor key (any one you choose) and you are going to borrow some chords from the major key. Look at the diatonic major and minor keys together to see what their harmonic differences are.
If you look at the chords, number by number, you can see differences; they are, in fact, different keys. Now, think about this: Discussions of minor keys typically include the addition of dominant chords on V and diminished chords on vii to make the keys function better. This is analogous to the discussion of the harmonic minor scale and its chords. In a sense, you have already borrowed the V and vii chords from the major key (see figure 12.9). The other most common chord to borrow is the IV chord from the major key.
FIGURE 12.9 Parallel Major and Minor Harmony
Here is an example in FIGURE 12.10, using a major IV chord in a minor key.
FIGURE 12.10 Borrowed IV Chord in Minor
You could certainly take any chord from the major key, but if you count IV, V, and vii°, those are three typical chords that are often seen in minor keys. Add to that secondary dominant and diminished chords, and you have a pretty nice lineup of harmonic choices.
You could even think of a Picardy third (a raised third that forms a major triad in the final chord of a composition in a minor key) as a borrowed I chord. Since it happens only at the end of pieces, it's not a true borrowed chord.
When you start looking at borrowing from the minor key into major, there are a lot more choices.
Borrowed Chords from Minor
In any major key, you can borrow a bunch of chords from the parallel minor key. Looking back at FIGURE 12.9, you can see the differences. The typical chords that are borrowed are iv, VII. In the jazz chapter (coming up next), you will hear about one other chord that is used in modern music, the ii°. For now, you should know about the iv, VI, and VII. Start with iv. The iv chord is very commonly used in major keys and it's all over music of all genres and ages. It's in Mozart and the Beatles. FIGURE 12.11 gives a nice example using a iv chord in a major key context.
FIGURE 12.11 Minor iv in Major
You don't even have to look that hard to find this borrowed chord. It's actually more common in folk and pop music than in common practice styles. It's all over the Beatles’ music!
The next two chords are the VII. In FIGURE 12.12, you see these particular chords, commonly ascending toward the I chord, typically coming between V and I as the example illustrates. There are other ways to use them, but this is by far the most common.
The reason to talk about borrowed chords at all (it's considered a fairly high-level subject) is that they occur often in all different styles of music, across genres and time periods. Most importantly, you see them in pop music, so this may eliminate some of the chords you've been trying to analyze that simply didn't seem to fit into the key.
FIGURE 12.12 Excuse Me, May I Borrow That VI and VII Chord, Please?