You may have heard about modal interchange in passing, but never had it explained. If you want to know what it is then you have come to the right place. I’m going to quickly break down for you what modal interchange is all about and what it can do for you in songwriting. In short Modal Interchange is the act of replacing chords in a harmonic progression with other chords belonging to parallel modes or scales to the key in which the song is written. Quite a mouthful, but don’t worry, it’s really easy. So if we’re replacing chords with ones that don’t belong in the key, aren’t we adding wrong notes? Why would we want to do that? Well, by using notes that don’t belong to the key that the song is written in you are adding what is known as “chromatic interest”, which is just a fancy way of saying that you’re adding the unexpected. To put it another way, you are ‘jazzing up’ your piece.
How It Works
There are two ways in which the major and minor keys (which are two classes of modes in the diatonic system) are related to each other, they can be relative or parallel to one another. Say we’re starting with a major key, the relative minor has a different tonic note than its major counterpart yet has the same key signature ( the pattern of sharps or flats). To start with the simplest example the key of C major has no sharps or flats, the key of A minor also has no sharps or flats. This makes A minor the relative minor of C major. In this diagram of the circle of fifths, the inner circle shows the relative minors of the outer circle.

Click to learn more about the Circle of Fifths.
Now as you can see if we were to try to use a relative key to borrow chords from it wouldn’t add any chromatic interest as you would not be introducing notes into your piece that aren’t already in the key in which it was written. In contrast, the parallel minor has the same tonic note yet different key signatures. In this case C minor is the parallel minor of C major. They both start on C, but the C minor scale has three flats (A-flat, B-flat, E-flat). On a quick side note: I’ve been speaking of parallel and relative minors, but the inverse relationship applies as well. C major is the parallel major of C minor and the relative major of A minor. Parallel and relative just describe the type of relationship two keys have. So remember: parallel – same tonic note, relative – same key signature.
That you use a parallel key to borrow from is the secret behind modal interchange. The pattern of intervals that make up a key is termed its mode. Of the seven modes (Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian, and Locrian) in common use in tonal music, the major scale corresponds to the Ionian mode and the natural minor scale corresponds to the Aeolian. So what you are doing by substituting chords and notes with those from a parallel key is literally interchanging the chords and notes between modes.
I think that I should be clear on something here. So far the discussion has limited itself to borrowing between two specific modes, the Ionian and the Aeolian. To be certain, there is no rule that you must limit yourself so in modal interchange. Indeed you can borrow from any mode you please with varying degrees of success. I suggest trying the Phrygian next.
Modal Interchange in Action
Let’s run through a quick example of what simple modal interchange might look like. Here is an easy progression in C major (Ionian mode):
C (I) / G (V) / F (IV) / Am (VI)
Play it through to see how it sounds. Now we’re going to make a slight alteration by replacing the G chord with it’s counterpart in the C minor scale (Aeolian mode). Play this one through and listen to how the tone of the progression changes.
C (I) / Gm (v) / F (IV) / Am (VI)
Notice how the progression becomes more ’serious’ and less ‘light’, yet we don’t have to leave the major key to obtain this effect (completely leaving one key for another would be modulation). By just including the B-flat in the Gm chord temporarily, we can avoid wandering into the ’sad’ sound of a full-on minor key. In a sense we are making the key less distinct without abandoning it completely. Thus modal interchange is a great way to spice up your song’s harmony; it becomes less predictable and more expressive than it would be if you stuck completely to one mode. In its own right, the natural experimentation with modal interchange will shake you out of the rut you’ve made for yourself of using the same chords over and over again. Think of it as a way to squeeze the creative juice out of your brain.
If you need some reference as to what chords go with what scales check out my chord transposition chart, and here is a great post on building progressions and charting the chords that belong to certain modes.