Plagal mode

Authentic and plagal

The system of the eight church modes arranges the four scales Dorian, Phrygian, Lydian and Mixolydian twice each. The authentic form moves from the finalis, the closing note, about an octave upwards. The plagal form has the same finalis, but its range reaches about a fourth below the finalis to the fifth above. The finalis then lies in the middle. Plagal modes carry the prefix “hypo-” and, in the medieval numbering, the even numbers two, four, six and eight.

In the glossary: Church mode · Finalis · Ambitus

How to tell the difference

The note collection is the same in both forms: Hypodorian consists of the same notes as Dorian. The forms are therefore distinguished not by the key signature but by two other features. First by the ambitus, the range of the melody. Second by the tenor or reciting note (repercussa), around which the melody circles and on which it is recited. In plagal modes it lies lower than in the authentic ones.

If you look only at the key signature (sharps or flats), you cannot tell authentic and plagal forms apart. In notation they are identical. Only the ambitus and the centre of gravity of the melody give information. The table shows the traditional tenors; in practice deviations and extensions of the range by a few notes are usual.

What it was for

For liturgical practice the distinction is essential, because each of the eight modes has its own psalm formula. An antiphon in the second mode calls for a different formula from one in the first, although both close on the same note.

Plagal in the literal sense

In another context the word “plagal” means something else: the plagal cadence denotes the progression from the subdominant to the tonic (IV–I) and has nothing to do with the ambitus. All they share is the Greek word, which means “oblique” or “sideways”.