Parallel chord substitutions

Question

Are parallel chord substitutions common in (classical) music (if so, please share some examples)? And do they have the same function as the original chords?

By saying parallel chords I am referring to the term Hugo Riemann used in his theory (Parallelklang in German). For example, in the key of C major (minor), the parallel chords for each chord degree are:

  I: A minor (Eb major)
 ii: F major (diminished, so none?)
iii: G major (C minor)
 IV: D minor (Ab major)
  V: E minor (Bb major)
 vi: C major (F minor)
VII: diminished, so none? (G minor)

Since any parallel chord for each chord degree is in the key of C major or C minor, I won't talk about them much (*).

But I was wondering, what if we use the parallel chord of, say, the Neapolitan, e.g. Bb minor, in the key of C? For example, instead of having the usual chord progression: I-iiN-V, we'd have: I-(iiN)p-V, where (iiN)p is the parallel chord of the Neapolitan, Bb minor. Please note that I am referring to V as function, so it could be V, V7, V+, VII7 (diminished or not), etc.

And also, what about secondary dominants? For example, in C major:

 V/ii: F# minor (instead of A major)
V/iii: G# minor (B major) (the same applies to the V/III)
 V/IV: A minor  (C major)
  V/V: B minor  (D major)
 V/vi: C# minor (E major)

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*I'd like to mention that, while writing the first table, I realized that the parallel chord of II is IV, and vice versa. Since we know that these two chords share the same function, does this answer my very question? Does this apply to the parallel chords of the "chromatic" chords as well (the Neapolitan and the various double dominants)?

However, interestingly, this does not apply to minor scales. For example, in C minor, the parallel chord of i is not Ab major (VI), etc.

Answer 1

Yes, you definitely started realizing your own answer at the end!

In your key of C major, the ii is the Parallelklang (what we call "relative key" in English) of IV, and vice-versa. And since those two chords both function as predominants, we do see a kinship between the two harmonies.

It's a similar story with I and vi: vi, the Parallelklang of I, often substitutes for I, especially at deceptive cadences where V moves to vi instead of to I. In more advanced tonal music, you'll occasionally see iii substituting for V, and those keys have the same relationship.

But while this is true, I think it's more important to realize that these two Parallelklang-related triads share 2 of 3 tones: C major and A minor both have C E, F major and D minor both have F A, and G major and E minor both have G B. In my mind, it's this relationship that allows these substitutions to occur. The fact that they are relative keys is, to me, less important and more coincidental than the statement "a chord that is 2/3 the same as another chord will in some sense sound similar." (Note also that the remaining pitch is only one whole step away from the pitch in the other triad.)

Looking at your second chart, I think you'd be really interested in reading about Neo-Riemannian music theory. As its name implies, it renews Riemannian ideas to help explain chord progressions that would be considered non-standard in tonal music. Just be aware that it was created by American scholars, so their terminology is based on the English language; this means that C major and A minor are relative keys while C major and C minor are parallel keys. Confusing, huh? :-)