IV. Diatonic Harmony, Tonicization, and Modulation
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.
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 dominant-function chords. ) or at the beginning of a phrase in place of the more common tonic prolongations involving
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 (
):- Make both I and (IV) complete (don’t omit the fifth).
- Double the bass (as is common in root position chords).
- 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.
Writing plagal motion at a phrase beginning
While the root-position version of plagal motion (as in
) also occurs at phrase beginnings sometimes, shows that (IV)