A borrowed chord (also called mode mixture and modal interchange) is a chord borrowed from the parallel key (minor or major scale with the same tonic). Borrowed chords are typically used as "color chords", providing variety through contrasting scale forms, major and the three forms of minor. Similarly chords may be borrowed from the parallel modes, the various modes beginning on the same tonic, for example Dorian or Phrygian.
Diminished supertonic triad (iio): D - F - A♭ Half-diminished supertonic seventh (iiØ7): D - F - A♭ - C "Flat three" (♭III): E♭ - G - B♭ Minor subdominant (iv): F - A♭ - C "Flat six" (♭VI): A♭ - C - E♭ Fully diminished leading-tone seventh (viio7): B - D - F - A♭
The following three chords are also found in Romantic era, albeit rarely:
Minor subdominant seventh (iv♭7): F - A♭ - C - E♭ Major-minor subdominant seventh (IV♭7): F - A - C - E♭ Dominant flat ninth (V♭9): G - B - D - F - A♭
The major-minor subdominant seventh, which contains an A♮, is borrowed from the parallel ascending melodic minor scale.
If the root of the borrowed chord is not in the original key, then the chord is named by the accidental. For instance, in a major key, a chord built on the parallel minor's sixth degree is a "flat six chord" written ♭VI.
Chord progressions may be constructed with borrowed chords, including two progressions common in rock music, I - ♭VII - ♭VI - ♭VII, common everywhere, and I - ♭VI - IV, used a lot by bands including Genesis, Yes and Nirvana. ♭VII is from Mixolydian and ♭VI is found in both Aeolian and Phrygian. The ♭VII-I cadence with ♭VII substituting for V is common, as well as ♭II-I, ♭III-I, and ♭VI-I. In popular music, the major triad on the lowered third scale degree (♭III), the major triad on the lowered sixth scale degree (♭VI) and the major triad on the lowered seventh scale degree, or "flat seven" (♭VII, in C major: B♭, D, F) are common. For more examples of borrowed chords in popular music, see list of songs with chromatic harmony.
A common borrowed chord from a parallel major key is the Picardy third.
In major the typical chords borrowed from minor are iv, ♭VI, ♭VII, and in jazz, the iio. The ♭VII is also known as the subtonic. The lowered-sixth occurs in many of the chords borrowed from minor and is a, "distinctive characteristic," of borrowed chords. Borrowed chords have typical inversions or common positions, for example iio6 and iiø, and progress in the same manner as the diatonic chords they replace except for ♭VI, which progresses to V(7).
See also 
- Romeo, Sheila (1999). Complete Rock Keyboard Method: Mastering Rock Keyboard, p.42. ISBN 0-88284-982-4.
- Benward & Saker (2009). Music in Theory and Practice, Vol II, p.71. ISBN 978-0-07-310188-0.
- Romeo (1999), p.43.
- Schonbrun, Marc (2006). The Everything Music Theory Book, p.138. ISBN 1-59337-652-9.
- "Mode Mixture", Robert J. Frank, Theory on the Web.
- "Glossary: Terms Used in Writing Skills", Berklee.
- Mike Campese. Guest Column: "Modal Interchange", Guitar Nine Records.
- "Altered Chords in Jazz: Modal Interchange - Altered Chords", Taming the Saxophone.