IV. Diatonic Harmony, Tonicization, and Modulation

Plagal Motion as a Form of Prolongation

John Peterson

Key Takeaways

  • Plagal motion from IV to I serves to prolong tonic either after an authentic cadence (AC) or at the beginning of a phrase.
  • We place plagal (IV) in parentheses in analysis to differentiate it from the more common use of IV as a predominant.
  • In 18th- and 19th-century music, this use of (IV) more commonly occurs as part of a prolongation rather than as part of a cadence, so this part of the book will use the term “plagal motion” rather than “plagal cadence.”
  • Writing with plagal motion is not difficult when both harmonies are in root position, but watch for parallels when either harmony is inverted.

Chapter Playlist

Earlier we saw that chords that function as predominants, such as ii and IV, are typically used to strengthen a phrase’s motion toward the dominant, often at the end of a phrase. In this chapter, we look at a special situation in which IV functions not as a predominant but rather to prolong I, which produces plagal motion. In analysis, we place plagal (IV) in parentheses to differentiate it from the more common use of IV as a predominant. Plagal motion most commonly serves to prolong tonic in two places: after an authentic cadence (as in Example 1) or at the beginning of a phrase in place of the more common tonic prolongations involving dominant-function chords.

Example 1. Plagal motion after a PAC in Handel, “Hallelujah Chorus,” from Messiah (3:02-3:29).

Plagal motion versus plagal cadence

You may sometimes see all (IV)–I motions described as “plagal cadences,” but this part of the book uses the broader term “plagal motion” instead, preserving our definition of cadences as marking the ends of phrases. While true phrase endings involving the progression (IV)–I do exist, in 18th-century classical music, it’s more common to see a (IV)–I tonic prolongation either after a PAC or at the beginning of a phrase. Using “plagal cadence” in all scenarios would inaccurately imply that (IV)–I always ends a phrase, whereas “plagal motion” allows us to describe the more common usage of (IV)–I as prolongational without rejecting the situations in which (IV)–I really does end a phrase.


Writing plagal motion after an authentic cadence

It’s most common to see both I and (IV) in root position when composers use plagal motion after an authentic cadence. The progression isn’t difficult to write, since the danger of parallels is low, provided you follow three pieces of advice (Example 2):

  1. Make both I and (IV) complete (don’t omit the fifth).
  2. Double the bass (as is common in root position chords).
  3. Move all upper voices by step or common tone.

It’s most common to have do ([latex]\hat{1}[/latex]) in the soprano, which makes sense given that plagal motion comes after a PAC where the melody has already completed its journey toward do ([latex]\hat{1}[/latex]). As always, follow typical writing procedures.

Example 2. Writing plagal motion with root-position chords.

Writing plagal motion at a phrase beginning

While the root-position version of plagal motion (as in Example 2) also occurs at phrase beginnings sometimes, Example 3 shows that (IV) can also go to I6. Here, watch for parallels between (IV) and I6: if the alto in Example 3 went to G (shown in parentheses) instead of B♭, it would create parallel octaves with the bass. Remember that it’s more common by far for tonic to be prolonged by an inverted V7 than by (IV) at a phrase beginning. When you see fami ([latex]\hat4-\hat3[/latex]) at the beginning of a phrase, your instinct should still be to use [latex]\mathrm{V^4_2-I^6}[/latex] rather than [latex]\mathrm{(IV)-I^6}[/latex].

Example 3. Writing plagal motion with an inverted tonic.

  1. Plagal Motion as a Form of Prolongation (.pdf, .docx). Asks students to identify bass lines and analyze an excerpt. Download audio (.mscz).


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