Using Modal Interchange Chords

It is a fact that the majority of popular Western music is diatonic. Therefore, it was composed using a fixed bank of available pitches that belong to one major or minor key signature. This system, however, is limiting.

Borrowing Weird Chords

In fact, thought-provoking and exciting harmonies are created by "borrowing" chords that are not diatonic to the song's key but to a parallel key. Furthermore, they are called modal interchange chords. They are evident in baroque, classical, and jazz music as well as in modern compositions from artists like Radiohead, Muse, Moby, Deadmau5, and Thelonious Monk. In addition, incorporating modal interchange chords will expand the "color pallet" of your music and the range of emotions you can evoke.

For Example.

If you begin a song in the key of F Major, the diatonic scale degrees are F, G, A, B♭, C, D, E. Correspondingly, the diatonic triads built upon these scale degrees are as follows...

F major (F A C)
G minor (G B♭ D)
A minor (A C E)
Bb major (B♭ D F)
C major (C E G)
D minor (D F A)
E diminished (E G B♭)

But You Can Use More than Just These 7 Chords.

Additionally, the parallel minor of F major is F minor. The two key signatures share a tonal center or "do." But, the 3rd, 6th, and 7th degrees in F natural minor are a half-step lower than in F major.

A becomes A♭
D becomes D♭
E becomes E♭

Diatonic Triads

Also, the diatonic triads upon the F natural minor scale degrees are as follows….

F minor (F A♭ C)
G diminished (G B♭ Db)
Ab major (A♭ C Eb)*
Bb minor (B♭ D♭ F)
C minor (C E♭ G)*
Db major (D♭ F A♭)
Eb major (E♭ G B♭)*

Minor

* In minor, diatonic triads that include the 7th degree of the scale are typically altered by raising that 7th degree a half-step (E♭ to E in this case). Consequently, the Ab major triad would become an A♭ augmented triad (A♭ C E), the C minor triad would become a C major triad (C E G), and the E♭ major triad would become an E diminished triad (E G B♭). This is an adaptation of classical harmony and the "Harmonic Minor" scale. Moreover, this raised seventh is called a "leading tone." It enables a sense of resolution by allowing the chord built upon the fifth degree to be major or dominant.

Roman Numeral Analysis

As can be seen below, I have isolated the triads from F natural minor that are most commonly interchanged for diatonic triads in F major, with their respective Roman numeral analysis**.

i: F minor -------> (interchanges I: F major)
iv: B♭ minor ----> (interchanges iv: B♭ major)
v: C minor -------> (interchanges V: C major)
♭VI: D♭ major --> (interchanges vi: D minor)
♭III+: Ab augmented-----> (interchanges iii: A minor)

** As shown above, Roman numeral analysis is a representation system standardizing chord functions. In like manner, modal interchange chords with the same Roman numeral analysis add the same "color" to the arrangement no matter what key you're in. For example……

Playing an F minor triad in the key of C major will feel the same as playing a C minor triad in the G major because the F minor and C minor chords function as the modal interchange chord, "iv," in their respective keys.
Playing a D minor triad in the key of D major will feel the same as playing a D♭ minor triad in the key of D♭ major because the D minor and D♭ minor chords function as the modal interchange chord, "i," in their respective keys.

In addition, you can also use "7th chords" from parallel keys. Some modal interchange chords are more effective as "7th chords" than triads, such as "♭II maj7".

Below are some more exotic modal interchange chords from the modes.

Roman Numeral	Mode of Origin	Example using F major
♭II maj7 (Neapolitan chord)	Phrygian	G♭ major 7 (G♭ B♭ D♭ F)
IV7	Dorian	B♭ Dominant 7 (B♭ D F A♭)
♭VII maj7	Dorian	E♭ major 7 (E♭ G B♭ D)
♯iv Ø	Lydian	B half diminished (B D♭ F A)
I7	Mixolydian	dominant 7 (F A C E♭)

*** In contemporary harmony, a flat or a sharp is placed before a Roman numeral to indicate that its root note has been raised or lowered about the major scale degrees.