V. Diatonic Harmony, Tonicization, and Modulation
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.
Plagal motion after a PAC in Handel, “Hallelujah Chorus,” from Messiah.
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):
- Make both I and (IV) complete (don’t omit the 5th)
- 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 () in the soprano, which makes sense given that plagal motion comes after a PAC where the melody has already completed its journey toward Do (). As always, follow
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 () in the bass going to Mi () at the beginning of a phrase, your instinct should still be to use V to I6 rather than (IV) to I6.
Writing plagal motion with an inverted tonic.
A cadence with the harmonies V–I. The harmonies are typically in root position. Authentic cadences can be further distinguished by their melody note in the I chord: an authentic cadence ending on scale-degree 1 in the melody is a perfect authentic cadence, while one with 3 or 5 in the melody is an imperfect authentic cadence.
Predominant function chords are those that transition away from tonic function toward dominant function. It's best to split this category into two groups: (1) Strong predominants are those that signal a dominant function chord is imminent. These are IV and ii (in minor: iv and iio). (2) Weak predominants are those that transition away from tonic, typically moving to a stronger predominant. These are iii and vi (in minor: VII, III, and VI).
Occurs when IV (or IV6) moves to I (or I6). Sometimes people have called this "plagal cadence," but we find that term too restrictive since plagal motion more often serves to prolong tonic than to create a cadence. The term "plagal motion" is more inclusive of the variety of contexts in which IV moves to I.
A melodic and harmonic goal. In classical tonal music, cadence types include Perfect Authentic (PAC), Imperfect Authentic (IAC), and Half (HC).
A relatively complete musical thought that exhibits trajectory toward a goal. In much music, that goal is a cadence; so we might also say that a phrase is a relatively complete musical thought that ends with a cadence.
1. Write the entire bass
2. Write the entire soprano to make a smooth melody that interacts well with the bass. Choose active notes for the soprano above dominant-function chords, and remember you need not write left to right always.
3. Write the inner voices by asking "what notes do I already have? What notes do I still need? Considering spacing and resolution, what note placement would give me the smoothest motion?"