A borrowed chord is a chord that is ‘borrowed’ from the parallel key. That is to say, if we are composing a song in C major, then we would borrow chords from C minor. Whilst not a confusing topic, there are a few things to note that might speed up the process of learning past trial and error.
It is more common for a major key to borrow from a minor key as a minor key already has lots of variation due to the many different types of minor (Melodic, Natural and Harmonic).
If a minor key were to borrow from a major key, it would not have much (if anything) to gain.
Below are the three common minor scales and there accompanying chords;
Please note: These are triads and not sevenths for the sake of simplicity.
m = Minor, M = Major
o = Diminished, += Augmented
A | B | C | D | E | F# | G#
m| M | + | M | M | o |o
A | B | C | D | E | F | G#
m| o | + | m | M | M |o
It should be noted that the most common borrowed chords from the minor key are:
ii - The diminished chord
iv - The minor chord
bvi - The major chord
This does not mean that you shouldn’t try other combinations like incorporating the augmented chord on the III of the harmonic minor scale or the two diminished chords at VI and VII of the melodic minor.
Food for thought: Borrowing chords can lead to an easier and smoother way to modulate to a different key.