V. Diatonic Harmony, Tonicization, and Modulation

Plagal Motion as a Form of Prolongation

John Peterson

Key Takeaways

This chapter introduces plagal motion [(IV)-I], which serves to prolong tonic either after an or at the beginning of a phrase. We choose to call it plagal motion as opposed to plagal cadence since the latter term restricts its use to cadences only when plagal motion more commonly occurs as part of a prolongation rather than as part of a cadence. Writing with plagal motion is not difficult when both harmonies are in root position, but watch for parallels when either harmony is inverted.

Overview: uses of plagal motion

Earlier we saw that chords that function is , 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 does not function as a pre-dominant. Example 1 shows how (IV) prolongs I after a phrase has ended with a PAC. We call this use of (IV) to prolong I, where IV goes to I, “.” Plagal motion most commonly serves to prolong tonic in two places: after an authentic cadence or at the beginning of a phrase in place of the more common tonic prolongations involving dominant-function chords. We place plagal (IV) in parentheses in analysis to differentiate it from the more common use of IV as a pre-dominant.

Example 1. Plagal motion after a PAC in Handel, “Hallelujah Chorus,” from Messiah.

Plagal motion versus plagal cadence

Some have described all (IV) to I motions as “plagal cadences,” but we find that description too restrictive. We prefer the term “plagal motion.” For us, always mark the ends of , and while true phrase endings involving the progression (IV) to I do exist, it’s more common to see a (IV) to I tonic prolongation either after a PAC or at the beginning of a phrase in place of the more common tonic prolongations covered in Beginnings 1 and Beginnings 2. We prefer the term “plagal motion,” then, because it allows us to describe the more common usage of (IV) to I as prolongational without rejecting the few situations in which (IV) to I really does end a phrase. The term plagal cadence implies that (IV) to I always ends a phrase, and that’s simply not the case.


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 5th)
  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 (\hat{1}) in the soprano, which makes sense given that plagal motion comes after a PAC where the melody has already completed its journey toward Do (\hat{1}). As always, follow

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 an 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 (see the note in parentheses in Example 3). Remember that it’s by far more common for tonic to be prolonged by an inverted V7 than by (IV) at a phrase beginning. When you see Fa (\hat{4}) in the bass going to Mi (\hat{3}) at the beginning of a phrase, your instinct should still be to use V\begin{smallmatrix}4\\2\end{smallmatrix} to I6 rather than (IV) to I6.

Example 3. Writing plagal motion with an inverted tonic.


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