Modal Interchange

Modal interchange means using a chord from a parallel mode, a scale with the same tonic but a different formula, while staying in your home key. The chord you take is called a borrowed chord.

Do not confuse this with the *relative* minor. C major and A minor are relative (same notes, different tonic). C major and C minor are parallel (same tonic, different notes). Borrowing always happens between parallel scales, because the tonic never moves. You hear the whole time that home is still C.

Here is the idea in one sound. In C major the chord on degree 4 is F (1 3 5 on F). Turn its major third into a minor third, A down to Ab, and you get Fm, the chord on degree 4 of C minor. Play C to F to C, then C to Fm to C. Same key, same tonic, one borrowed note, and the mood changes completely.

The parallel natural minor is where most borrowed chords come from, so let's build it on C and list every chord it contains.

Three notes changed (Eb, Ab, Bb), and every chord that contains one of them changes quality. Compare the two chord families side by side:

Read the numerals carefully: a b in front of a numeral means the *root* is lowered (bIII is built on Eb, not E); lowercase means the chord is minor or diminished. Every chord in the bottom row is available to a song in C major. The ones borrowed most often are iv, bVII, bVI and bIII. The next four sections take them one at a time.

The minor iv is the most-used borrowed chord in popular music: the IV of a major key played as a minor triad, taken from the parallel minor.

The classic move is IV-iv-I: F to Fm to C. Only one note changes between the first two chords, A falls to Ab, and then it falls again to G, the fifth of the tonic chord. That descending line A → Ab → G is the whole secret: it sounds bittersweet, like a sigh, and it lands on home.

• I-IV-iv-I: C F Fm C, the classic ending phrase C-F-Fm-C
• I-vi-IV-iv: C Am F Fm, a loop that gets darker at the end C-Am-F-Fm
• iv-I: Fm C on its own, a soft, minor-flavoured way to land on the tonic Fm-C

The bVII is a major triad built on the lowered 7th degree, a minor seventh above the tonic. In C, that is the chord Bb. It is the sound of classic rock, and it comes from two parallel modes at once.

Mixolydian is the major scale with only the 7th lowered: 1 2 3 4 5 6 b7 (C D E F G A Bb). It keeps the bright major 3 and 6, so bVII is a *bright* borrowed chord: a major key with one flat added. Natural minor contains the same Bb chord, so bVII also fits in the darker company of iv and bVI.

• I-bVII-IV-I: C Bb F C, the rock loop C-Bb-F-C
• bVII-IV-I: Bb F C, the roots fall by fourths into home Bb-F-C
• bVII-I: Bb C, the root climbs a major second into the tonic Bb-C

That last move deserves a name. bVII-I is step-wise root motion: Bb rises a major second to C. There is no leading tone (B) in it, so it does not push like V-I; it arrives from below and settles. Jazz players call bVII7-I (Bb7 to C) the backdoor resolution, a way into the tonic that avoids the front door of G7.

The bVI is a major triad on the lowered 6th degree, a minor sixth above the tonic. In C that is Ab, and it is the most dramatic of the common borrowed chords.

Going from C to Ab, the tonic note C stays put while E slides down to Eb and G slides up to Ab. Two notes move by a semitone in opposite directions around a common tone. That is why the chord feels like the floor tilting, not like a mistake.

• I-bVI-bVII-I: C Ab Bb C, the "epic ending": two borrowed chords climbing home a major second at a time C-Ab-Bb-C
• I-bVI-IV-V: C Ab F G, bVI as a surprise in place of vi C-Ab-F-G
• I-V-bVI-IV: C G Ab F, the everyday I-V-vi-IV loop with the vi darkened to bVI C-G-Am-FC-G-Ab-F

The bIII is a major triad on the lowered 3rd degree, a minor third above the tonic: Eb in C. It is the tonic chord of the relative major of C minor, which is why borrowing it makes a major key sound suddenly heroic and a little wild.

bIII, bVI and bVII are the three *major* chords of the parallel minor, and they love to travel together. The roots Eb, Ab and Bb are all foreign to C major, so a run of them sounds like the song has changed colour without ever leaving C.

• I-bIII-IV-I: C Eb F C, the bIII pushes up a major second into IV C-Eb-F-C
• I-bIII-bVII-IV: C Eb Bb F, four major chords, two of them borrowed C-Eb-Bb-F
• bVI-bVII-I: Ab Bb C, the climb: major second, major second, home Ab-Bb-C

The climb bVI-bVII-I is worth memorising as a unit. Each root rises a major second, so the ear hears a staircase that arrives at the tonic from below. It is the borrowed cousin of the IV-V-I cadence, and just as final.

Borrowing is not only about major chords. The parallel minor also lends its ii°, the diminished triad on degree 2, which turns the ordinary ii-V-I into something darker and more urgent.

Again only one note changes: the A of Dm becomes Ab, a perfect fifth above D shrinking to a diminished fifth. Add the seventh and Dm7 (1 b3 5 b7) becomes `Dm7b5` (1 b3 b5 b7, D F Ab C), the half-diminished chord you met in the seventh-chords chapter. Both lead to G7 exactly as ii does. The Ab simply adds extra pull, because it wants to fall onto the G right below it.

Borrowing works in the other direction too. A minor key can brighten one chord by taking it from the parallel Dorian mode, and the chord it usually takes is a major IV.

The only difference between C Dorian and C natural minor is the A in place of Ab: a major sixth above C instead of a minor sixth. That one note sits inside the chord on degree 4: Fm (F Ab C) becomes `F` (F A C). So in a song in C minor, Cm to F to Cm is a Dorian borrow: the IV is major even though the key is minor.

This is the exact mirror image of section 3. There, a major key borrowed iv from its parallel minor; here, a minor key borrows IV from its parallel Dorian. Same two chords, F and Fm, same single note A/Ab. The direction of the borrow depends only on which one is home.

• i-IV-i: Cm F Cm, the hopeful lift of folk and modal rock Cm-F-Cm
• i-IV-bVII-i: Cm F Bb Cm, Dorian IV next to the minor key's own bVII Cm-F-Bb-Cm
• i-IV: Cm F repeated, a two-chord groove that never resolves Cm-F

The parallel Phrygian mode lends a chord that lives a minor second above the tonic: the bII, known in classical harmony as the Neapolitan chord.

The chord on b2 is a major triad: bII = 1 3 5 on Db, that is Db F Ab. Classical composers almost always play it in first inversion, `Db/F` with F in the bass, so the bass can step up to G for the dominant: Db/F to G to Cm. The top note Db then falls to C (or, through the B of the G chord, to C), and the whole cadence has a dark, formal grandeur.

In rock, metal and flamenco the same chord is used raw, in root position and going straight back to the tonic: Cm Db Cm, or even C Db C in a major key. The minor second between the two roots is what gives Phrygian its Spanish, brooding character.

• bII-V-i: Db/F G Cm, the Neapolitan cadence Db/F-G-Cm
• i-bII-i: Cm Db Cm, Phrygian colour, roots a minor second apart Cm-Db-Cm
• I-bII-I: C Db C, the same colour dropped into a major key C-Db-C

The parallel Lydian mode is the only mode brighter than major, and it lends a major II chord, D in C, that floats instead of pushing.

The raised F# sits inside the chord on degree 2, so Dm (D F A) becomes `D` (D F# A). Played C D C, the D chord shimmers: the F# is the brightest note you can add to C, and the progression never leaves the neighbourhood of the tonic.

One more Lydian chord is worth knowing: the diminished triad on #4, F#dim (F# A C). Outside Lydian it is best understood not as a borrowed chord but as a chromatic neighbour, a passing chord that walks the bass from F up to G: F to F#dim to G.

Once a progression borrows once, it can borrow again. Chords from the same parallel mode fit together naturally, because they share the same borrowed notes.

Take I-bVII-iv-I: C Bb Fm C. Both borrowed chords come from C minor, and they share the note F. Bb to Fm even sounds like a small IV-i inside the minor mode before Fm sighs back into C. Now take I-IV-iv-bVII-I: C F Fm Bb C. Here F darkens to Fm, the Fm moves to Bb (roots a fourth apart, a strong move), and Bb climbs the major second home.

Step back and the whole chapter is one picture: a single tonic, C, surrounded by every mode that can be built on it. Each mode is a different formula, and each formula offers chords the others do not.

Read the table top to bottom and the formulas gain one flat at a time: #4 disappears, then 7 becomes b7, then 3, then 6, then 2. That is the same order of brightness you met in the modes chapter, and it tells you, before you play a note, that II is the brightest borrow and bII the darkest.

Every borrowed chord in this chapter eventually did one thing: it found a new way to arrive at C. iv-I, bVII-I, bVI-bVII-I, bII-V-i: these are all cadences, the arrival formulas of harmony. The next chapter names them, compares them with the classic V-I and IV-I, and shows how a cadence tells the listener whether a phrase is finished.