Audio: borrowed chord (0:08)
borrowed chord plays borrowed chords on a concertina. The borrowed chord figure shows the score:
A parallel key has the same tonic note as the prevailing key.
Modal interchange is the technical name for using a borrowed chord. It is a form of short-term modulation or tonicisation. Modal interchange generally lasts a beat or two and no longer than a bar. The effect is always of an abrupt change in key, not a gradual change, because parallel keys are distant keys and are not closely related to the prevailing key.
Modal interchange is used to write the hook in chorus. The progression F#m-F in the key of A, vi-bVI, forms the hook. The bVI chord is chromatic in the key of A but diatonic in the parallel key of A natural minor (and A phrygian and A locrian too). Borrowing the F chord from A natural minor to use in A major makes the key briefly modulate from major to minor and provides the contrast in the hook.
Modal interchange allows you to write all sorts of different chord progressions.
Modal interchange justifies replacing the I, IV and V chords in a major key with their minor counterparts i, iv and v. And vice versa, a minor key can always borrow a I, IV or V from a major key. In the key of C major, for example, the tonic triad I can be replaced with a chord borrowed from any of the parallel modes. A C minor chord, CEbG, can replace the C major chord, CEG, because i can be borrowed from the parallel key of C natural minor (or C phrygian and C dorian). More unusual is to replace I or i with io, a diminished triad from the parallel key of C locrian, as shown in the second bar of borrowed chord.
Modal interchange enables you to justify many but not all chord progressions. Take, for example, the chord progression C-D-E in bars 3-4 of borrowed chord. This can be justified using modal interchange as a bVI-bVII-I progression in the key of E major with the bVI and bVII chords borrowed from the parallel key of E natural minor. However, C-D-E cannot be constructed in the key of C using modal interchange. The D chord can be justified, because it represents II in the parallel mode of C lydian, but the E chord cannot. There is no parallel key in C that uses this chord. Although there is a III in C aeolian, C dorian and C phrygian, these all produce an Eb chord not an E chord.
It is worth repeating that a chord needs no justification in pop harmony. Justifying the use of a chord is a feature of functional harmony, not pop harmony. Pop music is nonfunctional. A chord just is, it has no purpose other than to sound pleasing. A chord that sounds pleasing to you needs no further justification: if it works, use it.
As an aside, and for what it is worth, you will always find a way in any type of harmony to justify the use of any chord. Eventually.