plagal

Plagal is an adjective used in music theory to describe a relationship or motion that involves subdominant-to-tonic cadence concepts, or to describe a class of church modes known as the plagal modes. The term derives from Greek plagios, meaning slanted or oblique, reflecting the perceived relation of the tonal system to its final center.

In medieval church music, the eight modes are divided into authentic and plagal groups. Plagal modes begin on a fourth below their final and typically have a range that extends from a fourth below the final up to a fifth above. The four plagal finals are D, E, F, and G, corresponding to Hypodorian, Hypophrygian, Hypolydian, and Hypomixolydian. This labeling contrasts with the authentic modes, which begin on the final and generally do not reach as far below it.

In tonal harmony, a plagal cadence describes a progression from IV to I, a movement from the subdominant to the tonic. This plagal cadence is common in hymnody and sacred music and is often referred to as the Amen cadence. It typically has a softer or less final quality than the authentic V–I cadence, though it can serve as a conclusive ending in many contexts.

Outside strict harmonic theory, plagal can describe any oblique or slanted relationship to a tonal center, but its primary technical usage is in reference to the subdominant-to-tonic cadence and the associated modes.