Plagal Motion as a Form of Prolongation
Plagal motion is movement by a perfect fourth downward (or fifth upward) in the bass, most famously the progression IV–I ("amen" cadence). Unlike authentic motion (V–I), plagal progressions do not involve a leading tone and carry less harmonic tension — yet they are pervasive in tonal music, especially in hymnody, folk music, and the closing gestures of Classical works. The plagal cadence (IV–I) can end a piece or a phrase, but it more often extends or confirms an already-established tonic.
Plagal motion can prolong the tonic in several ways. A common pattern appends IV–I after an authentic cadence, expanding the final tonic with a gentle plagal echo. Alternatively, IV–I can appear at the opening of a phrase to ground the tonic before any predominant–dominant motion begins. In both cases the IV does not function as a strong predominant driving toward V; instead it moves directly to I, bypassing the dominant entirely and creating a more stable, subdued harmonic atmosphere.
In minor keys, iv–i plagal motion (minor subdominant to minor tonic) is especially characteristic: the flat 6̂ (le) — the chordal third of iv — creates a dark, modal color, resolving down to 5̂ as the bass moves from 4̂ to the tonic. The IV (or iv) can also neighbor or pass between two positions of I, functioning as a subdominant neighbor chord. Recognizing plagal patterns — by their fourth/fifth bass motion and the absence of a leading-tone approach to the tonic — distinguishes this gentle, affirmative harmonic world from the urgency of dominant-driven progressions.
