What is a borrowed chord?
C major and C minor share a tonic but not a key signature: C minor has three flats, B♭, E♭ and A♭. A borrowed chord is a chord built from the C minor scale and played inside a piece in C major, so the piece stays in C while one chord brings a note that C major does not have. Classical theory calls this modal mixture; jazz and pop call it modal interchange. Two keys that share a tonic are called parallel. Do not confuse C minor with A minor, the relative minor, which shares the key signature but not the tonic.
Why do borrowed chords have flats in their numerals?
Roman numerals are counted from the major scale of the key, so each numeral says where the chord’s root sits against C major. The third degree of C major is E; a chord rooted on E♭ is ♭III, with the flat in front. The same goes for ♭VI (A♭) and ♭VII (B♭). The fourth degree needs no flat, because F is in both scales; its chord is minor, so it is written in lower case, iv. Case still means quality: a capital for a major third, lower case for a minor third.
Which chords does C major borrow from C minor?
Stack thirds on the notes of C minor and four chords stand out, because each holds a note outside C major. They are the ones the practice tool asks, written here as seventh chords. In a minor key these four are the key’s own chords, since the flats are already in its scale. They can be used one at a time or in a run from the tonic, as the practice tool plays it: Imaj7, ♭IIImaj7, ♭VImaj7, ♭VII7, which in C is Cmaj7, E♭maj7, A♭maj7 and B♭7.
| Numeral | Chord in C | Notes | Not in C major |
|---|---|---|---|
| iv7 | Fm7 | F A♭ C E♭ | A♭, E♭ |
| ♭IIImaj7 | E♭maj7 | E♭ G B♭ D | E♭, B♭ |
| ♭VImaj7 | A♭maj7 | A♭ C E♭ G | A♭, E♭ |
| ♭VII7 | B♭7 | B♭ D F A♭ | B♭, A♭ |
What does the minor iv sound like?
It is the most borrowed chord of all. Swap the major IV for the minor iv and the tonic stays major, but a shadow crosses it: A♭ is a note C major does not have, it sits just above G, the fifth of the tonic, and it leans down to it. Ballads and pop songs often use it just before returning to I. Compare Fmaj7 with Fm7: only two notes change, A drops to A♭ and E drops to E♭.
What is the backdoor dominant?
The chord ♭VII7, B♭7 in C, has the shape of a dominant seventh, a major triad with a flat seventh, yet it is not the dominant of C. It is a borrowed chord built on the lowered seventh degree, B♭, and its move to I is called the backdoor. The ordinary V7 comes in at the front, its root falling a fifth to the tonic; the backdoor comes in from a whole step below. Its notes lead down into the tonic chord: A♭ falls a half step to G, F falls a half step to E, and D falls a whole step to C. The pull is gentler than that of V7, which has a leading tone: nothing in B♭7 sits a half step below C.
It often follows the borrowed iv7. Fm7, B♭7, Cmaj7 is called the backdoor ii–V, because Fm7–B♭7 is in origin the ii–V of E♭, the ♭III, turned aside to land on C. In the practice tool that is written iv7, ♭VII7, Imaj7.
How is a borrowed chord different from a secondary dominant?
Both bring in a note from outside the key, and both are written with an extra sign, but they answer different questions. A secondary dominant asks where the chord goes: D7 in C major is the dominant of G, a fifth above its target, and the F♯ it brings points at G. A borrowed chord asks where it comes from: Fm7 is from C minor, the same tonic in the other mode, and it is heard as C with a darker colour rather than as a visit to another chord. The look-alike is B♭7. It is a dominant seventh in shape, but its target would be E♭, and in C major it goes to C, so it is borrowed, not a secondary dominant.