Why do parallel keys matter?

Question

I am learning about borrowed chords and all explanations state it's done in conjunction with a parallel key.

Why?

Wikipedia says: "In the early nineteenth century, composers began to experiment with freely borrowing chords from the parallel key."

https://en.wikipedia.org/wiki/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:

1. C# ... borrowing a C-sharp half-diminished 7th chord .. C#, E, G, B.
2. 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?

Answer 1

I am learning about borrowed chords and all explanations state it's done in conjunction with a parallel key.

Why?

Because that's what "borrowed chords" are, chords from the parallel key. It's literally just a definitional statement.

But why did they do that?

I can take a few guesses, but you need to understand the truth is "because they liked the sound". The only reason we copied them is because we also liked the sound. Theoretical justifications are post-hoc rationalizations, they are systems which describe existing examples, not immutable laws.

The most obvious guess is that they were just extending an older example from folk music, the Picardy third, major-tonics used to end otherwise minor-key phrases. The minor key already played loose with sixths and sevenths, so in a sense minor-key music was "already" "allowed" to borrow chords from parallel major very liberally. It's not really a huge jump to go from there to borrowing parallel minor chords in major.

To try and answer a little "more theoretically", the problem with borrowing accidentals from neighbouring keys is that you're very likely to end up modulating. Borrowing chords from the parallel minor means borrowing chords that relate to the same tonal centre you already have.

My first thought would be to go to the adjacent keys and play a chord starting on the different accidentals between the two keys.

Go do that if you want, music theory isn't your dad. As I've hinted at, and other answers have pointed out, this is generally going to give you something else that we already have a word for, "secondary dominants".

The point again being that this method has some sense to it

It looks kinda like it does to our pattern-recognition-obsessed brains, but it doesn't actually hold up to any academic rigour; you cannot outline an objective argument that makes this "the most sensible way to take chords from outside a key". You've just learned that the circle of fifths is important and gone "well I guess that's where I'd want to borrow chords from". You could apply the same "fifths are important" superstition to melody writing or chord construction and get results which would not at all resemble the way we tend to write music. On some level, that's okay, there's no reason you can't write a melody that revolves primarily around jumping between fifths, or a piece based around quintal harmony; it's no more or less "sensible" than the way we currently do things. Again, music theory is post-hoc rationalization of existing examples, none of this is "real".

I am asking specifically why moving to a parallel key was denoted as borrowing given so many other better options such as adjacent keys

It's just terminology, why do we call birds "birds"? You must rationally understand, as someone who speaks a language, that we just stick labels on things. Those labels aren't special.

Answer 2

Bit of a strange question - why do they matter?

It's a straightforward move from one parallel key to its partner. Same root, same P4, same P5. It changes the mood without having to move very far for all instruments - if they use the maj/min equivalents. But that's not that often.

The listener feels where 'home' is, and that is static. It's only the mode of transport that's changed.

Modulations can and do occur to all sorts of different keys - subdominant and dominant are common, the key change up a semitone or tone also.

It's an option that works (well) and gets to the maj/min in a different way than it would by using relatives - another common ploy.

Exploring new options moves music on, and I guess the first composers to do the parallel trick were doing just that - finding something new. It worked, thus it was used a lot.

Answer 3

Think of it this way. If you’re writing a vaguely classical thing in C major and to round off a phrase you use the chord sequence C Am D7 G then the f-sharp in the D7 is a chromatic alteration needed to temporarily tonicise or modulate to G major. This is not modal borrowing - the D major chord is not alluding to C Lydian. If you instead decide to end your phrase with a more 19thC C F Fm G then that Fm chord is a borrowing from the parallel minor (instead of ‘borrowing’ you will also read about ‘modal inflection’). To give your phrase ending a 70s folk ballad vibe it might go C Gm B-flat F where the G minor and B-flat chords could be interpreted as borrowings from C mixolydian. In tonal music, borrowings from parallel keys are more common than borrowings from other keys although i don’t think that more ‘important’ is a helpful thing to say. A jazz turnaround to the same phrase might go C-E7-Ab9-Db7 but describing the last three chords as modal borrowings in the above sense won't be possible or helpful - you’ll need different vocabulary and different theory.

Answer 4

Modulations or the movement from one key to another is common because music makes us move. There is a reason dance and music has a unavoidable connection. If it's when you go to a rock-concert and feel the stack of Marshall's pump air against your chest or whether you are emotional touched by a Chopin's nocturne, all music leads to movement.

Modulations is how you harmonically speaking move your music. You go to the parallel, because it is an easy way to strike a contrast in emotions. Chances are good if you hear a sad song, with a hopeful part in the middle, a modulation to the parallel key gives that effect, this all leads to effective movement and good conveying of emotions. Things that all music, and largely all art aims (or should aim) to do.

Here is a example that comes to mind how a modulation to the parallel key, listen how going to the major gives a hopeful effect to the bridge of an otherwise sad song.

Answer 5

Composers already use your first example of "borrowing" plenty of times. They just don't call that borrowing from a parallel key.

Chords like F♯-A-C-E in C major are called secondary dominants. Due to using the leading tone of the dominant of the home key, they strongly tend to resolve to the dominant. They can be found in Baroque music, so I presume composers got bored and wanted to borrow from more exotic keys.

Your example chord C♯-E-G-B is another secondary dominant - this time a secondary dominant of the ii key/chord or the V/V key/chord. C♯-E-G-B in C major often can be expressed as vii half-diminished 7/ii or vii half-diminished 7/V/V in Roman numerals.

Your examples of borrowing with flats can be interpreted as borrowing from the parallel minor. The chords B♭-D-F-A and E♭-G-B♭-D can be found in C minor.

The parallel keys are often used as umbrella keys to borrow from, partially because you can borrow so many chords from them as opposed to, say, the subdominant key.

Answer 6

I didn’t know the term borrowed chord before I came on this SE site, but I knew and practiced the minor subdominant from jazz and pop songs and also the major IV in the dorian scale or the major V of harmonic minor tunes. I was also familiar with the mediants that are called here parallel chords. The major II was a variant of the minor ii, the major bVII could be derived of the aeolian minor mode and the Catalonian cadence.

Now this part of my reflections could already answer the history of the development of the use of borrowed chords.

But my spontaneous reaction was thinking about the early Renaissance period when there were no tonic and no dominant chords but the same progressions of chords were in use by the common keys and chord repertory of that time before the well-tempered tuning: F->A->C->D

Thus I googled borrowed chords and back to Renaissance and found this site which supports pretty well my own assumptions:

http://www.practicalmusictheorybook.com/Borrowed_Chords.html