How do I correctly borrow chords from one key to another?

Question

So how does one decide to borrow chords? Let's say I am in the key of C Major. Can I borrow chords, say, from the G Major key? How does this work?

Answer 1

There are many ways to borrow from other keys. The most common way and the one I will deal here with here because anything else would require writing a book, is parallel keys that have the same route as was referenced before. But the reference does not have to be to the route it can be to any chord in a progression that matches the root of a parallel key. The matching is a pivot point. And example might be playing C, in C Major, to an F minor to a Bb dominant resolving to a C. The F is the pivot chord since the F root exists in both the key of C and the key of Eb; but, we change the quality. The chord Bb7 does not exist in the key of C and is being used as a what is called a backdoor dominant because it shares three notes (D,F,Ab) with the diminished chord which substitutes for the G7. If resolved correctly, the Bb note would resolve as a #9.

The point is that you are not required to commit to the entire parallel key to borrow from it.

Answer 2

When we talk about borrowing chords, we are typically talking about borrowing from another mode that has the same tonic, not just any another key. So if I'm in C major, the chords I'm borrowing are probably from C minor: using F minor (iv) instead of F major (IV), for instance.

However, any chord in C major can also become a sort of temporary tonic, and bring with it chords from its own key. This is often what's happening when you see a D major chord in C major: G (V in C major) is being temporarily tonicized, and so the D major chord is acting as a V chord in the key of G. We give that chord the Roman numeral V/V (five of five). But a chord like that is usually called an applied dominant or a secondary dominant. The term "borrowed chord" is reserved for situations such as the one I described in the 1st paragraph.

Hope that helps!