I am learning about borrowed chords and all explanations state it's done in conjunction with a parallel key.
Wikipedia says: "In the early nineteenth century, composers began to experiment with freely borrowing chords from the parallel key."
But why did they do that?
My first thought would be to go to the adjacent keys and play a chord starting on the different accidentals between the two keys.
Going up it would be the 7th note (vii in the dominant key; flat-5 in the main key) and going down a key it would be the iv the dominant key; flat-7 in the main key).
For example, for Cmajor the next key up is G. The accidental is F# ... the vii in G, the flat-v of Cmajor. The next key down is F. The accidental is B-flat ... the iv in F, the flat-vii of Cmajor.
So the first borrowed chord will be F# half-dim chord in G ... the 7th being F#, A, C, E.
The second borrowed chord will be B-flat Major 7 ... the 7th being Bb, D, F, A.
If this method was repeated and the next adjacent keys were used to borrow chords (D and B-flat) then the accidentals the borrowed chords are built upon will change to:
- C# ... borrowing a C-sharp half-diminished 7th chord .. C#, E, G, B.
- E-flat major 7th (Eb, G, Bb, D)
The point again being that this method has some sense to it whereas I don't see the reason to look for chords in the parallel key.
Is there a reason they did that? That is the question.
After all isn't there a 7th chord for every note that can be borrowed in any key because one is in-effect borrowing major and minor chords without regard to their tonic note?