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I have heard that going in between 2 parallel keys is just modal interchange and not a modulation because the tonic note is the same. Maybe if the change in key is short and only to add color to a piece that is clearly in a single key. But if there is a clear cadence in the new key or the passage in the new key is extended past simply adding color, I would argue that it is indeed a modulation. Especially if the parallel keys are major and minor(which is the most common set of parallel keys). For demonstration purposes I will use quotes and for actual examples, I will use classical music pieces that I know.

Modal Interchange:

C, F, G, Am, C ... | C, Cm, F, G, Ab, C | Back to usual chord progression

That I would consider to be modal interchange between C major and C minor. No modulation because the cadences that exist here are all in C major. No cadences to C minor at all, just sudden shifts for the purpose of adding color. Now for an example of this in action:

The sudden C minor chord out of nowhere doesn't last long before it goes back to C major and there is no cadence to C minor so clearly, this is just modal interchange and the only real modulation going on there where the C minor chord appears is a modulation to E minor.

Parallel Modulation:

C, F, Am, G, C, ... usual progression in C major | C, Cm, Fm, Ab, Bdim7, G, Cm | Continues in C minor for long enough that C minor sounds tonicized

This I would definitely consider to be a true modulation to the parallel minor. It continues on in C minor as though C minor was the starting key all along. And there is a clear cadence in C minor. Both of these point towards a true key change, a true modulation, and not just C minor being used for musical color.

Now for an example of this happening:

There are multiple motions here from G to Cm in the Scherzo section. And then the C major comes out of nowhere the same way the C minor does. This, I would argue is parallel modulation because an entire section is in C minor, not just a few chords.

So is parallel modulation really a thing? Or is it just modal interchange, even if the parallel key gets tonicized?

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It's clear enough what you mean by "parallel modulation" but the textbook definition of modulation is a change of tonal center, a change of tonic. It seems change of mode applies regardless of the extent of mode change.

This seems merely a matter of terminology.

I suppose if you want a way to distinguish between the extent of the change - like modulation versus tonicization - you might refer to borrowed chords for short passages and a mode change for something longer.

The only other thing I can think to add is: rather than look at the (mode) change of the tonic chord, ask if the dominant chord has changed. If the dominant hasn't changed, no change of tonic, no modulation (or tonicization.)

  • I take some slight issue with your 4th paragraph. Why would you argue that the dominant being unaltered precludes a modulation? C major and C minor can both use G major as a dominant, so why would using G force the progression to be labelled modal mixture rather than a modulation? – user45266 Jun 12 at 4:15
  • @user45266, with a G dominant there is only one tonic C - the tonic chord being either major or minor. No change of tonal center, no modulation. The point is to not get fixated on the tonic, but understand how strongly dominants determine tonality. – Michael Curtis Jun 12 at 12:35
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A change from a key to the parallel key is a modulation, a so-called "parallel key modulation" or maybe just "parallel modulation" just as your headline says. You can google "parallel key modulation" and find more information.

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