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In typical 19th-century harmony, was a Plagal Cadence always used with an Authentic Cadence preceding it? To put it another way, was it typically ever used out of its "Amen" context?

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JS Bach seemed to do everything under the Sun in his 371 Harmonized Chorales. This opening phrase caught my eyes several months ago, because it appears to me to end with a plagal cadence...

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If phrases end with cadences, and the fermatas in chorales mark those phrases, is there any reason to not regard that first passage a phrase with an ending cadence?

If it is a cadence, and the harmony is clearly IV I, is there a reason to not regard it as a plagal cadence?

Of course I'm not directing those questions to you, they are rhetorical. I think this must be regarded as a plagal cadence.

Some might say the passage is an elaboration of I IV I. But, IMO that's just a rationalization to not call it a plagal cadence. The sequential [I V] ... [IV I] and secondary dominant to IV are too much to gloss over.

Stylistically the chorales touch on older harmony and aren't the periodic and sonata structures of the later classical style which focused on tonic/dominant harmony. It's in the classical style that plagal cadences aren't considered proper cadences. JS Bach was famously a transitional composer between the old and new classical style. So this example represents the older style with different sensibilities about harmony and cadences.

Personally, I think the so-called "Amen" plagal cadences aren't good examples of cadences. They are codas. You can probably find better examples of real plagal cadences, ending real phrases. But you probably won't find many from the classical style.

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    It's worth noting that these intermediate cadences are melodically cadencing on the dominant, even if Bach has chosen a harmony other than the harmonic dominant. It would not be unreasonable to call them half cadences, at least in their structural function, even though they do not fit the typical harmonic model of a half cadence. Whether that is consistent with also calling the first one "plagal" could probably be argued either way.
    – phoog
    Jun 16 at 17:20
  • @phoog, I did notice that, and that it arrived at the dominant as a descent from the tonic, although not in a straight scale line. (I had been looking for harmonizations of a descending tetrachord in soprano.) I asked about it here music.stackexchange.com/questions/109822 but my idea was a bit far fetched. Jun 16 at 17:51
  • But, in terms of cadence types, I think of these as good examples of imperfect, in the sense that the chord is tonic, but the soprano isn't on ^1, to sent up continuations, rather than final endings. Jun 16 at 17:55
  • It just occurred to me that it would not be unreasonable to consider this a half cadence in IV, that is, to analyze the E flat chord as V/IV. I haven't really thought it through; I'm just brainstorming really, as I've stumbled on this question again through a web search for information about Bach's tendency to move to the subdominant to signal that a piece is approaching its final dominant-tonic cadence. Perhaps the ambiguity isn't surprising, considering that we're analyzing this in a theoretical framework that arose after Bach's time.
    – phoog
    Sep 4 at 19:25
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The nineteenth century is very broad; it had some of Haydn, Wagner, and Debussy!

But certainly in the first half of that century, the authentic cadence (V to I, both in root position) was supreme and treated as the "true" "form-defining" cadence. Plagal cadences (or "plagal motions," as some call them, to hint that they're not true cadences per se) typically function as what we call "post-cadential," meaning they occur after the "real" cadence of V–I that happened somewhere earlier in the piece.

But starting somewhere in the middle of that century—maybe two thirds of the way through?—we start to see composers using that plagal motion more and more often, and occasionally those moments are even used to end entire sections where there don't seem to be any authentic cadences in the immediate vicinity.

The conclusion of Wagner's Tristan und Isolde (completed 1859), for instance, ends not with a V–I motion, but with a series of plagal moves (as does his Ring cycle, completed in 1874). One can certainly argue that there are prior V–I motions that may be the "true" cadences, but nevertheless this shows the slow change in hierarchy of these cadential types.

This further evolved into practices like we see today: where IV to I can be understood as a proper cadence (as can any other number of chord progressions), depending on the style of music.

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