Plagal mode

What it means

A plagal mode is a mode whose tonal space extends substantially below the finalis, instead of lying predominantly above it. To every authentic mode belongs a plagal one with the same finalis; its name is formed with the prefix “hypo-” — Hypodorian, Hypophrygian, Hypolydian, Hypomixolydian.

Musical use

Authentic and plagal forms are distinguished not by the stock of notes but by two other features. First, the ambitus: the authentic mode moves roughly from the finalis an octave upward, the plagal one roughly a fourth below the finalis up to the fifth above it. Second, the reciting tone, around which the melody circles and on which it recites; it lies differently in the two forms.

For liturgical practice the distinction is essential, because a psalm formula of its own belongs to each of the eight tones. An antiphon in the second tone demands a different formula than one in the first, although both close on the same note.

Whoever looks only at the key signature cannot distinguish authentic and plagal forms — they are identical in the notation. Only the ambitus and the centre of gravity of the melody give information.

Bear in mind that the word “plagal” means something different in another context: the plagal cadence designates the turn from the subdominant to the tonic and has nothing to do with the ambitus.

The plagal modes carry the prefix hypo-, for instance Hypodorian or Hypophrygian. Their tonal space reaches roughly a fourth below the finalis and a fifth above it, so that the finalis lies in the middle. The reciting tone lies lower than in the authentic mode. In the medieval numbering, the plagal modes carry the even numbers two, four, six, and eight.

Performance practice

With modal music, always determine both — finalis and ambitus. The register tells you where the melody has its centre of gravity and how it is to be phrased. And check, with liturgical chants, which psalm formula is assigned; it follows the tone, not only the closing note.