Authentic mode

The range decides

Every church mode has a final note, the finalis. Two modes can have the same finalis and yet differ in the space in which the melody moves, the ambitus. In an authentic mode this space lies above the finalis: from the finalis up to the octave above. The melody thus rises from the final note and returns to it. In the doctrine of the eight church modes these are modes 1, 3, 5 and 7: Dorian, Phrygian, Lydian and Mixolydian.

Authentic and plagal

Every authentic mode has a plagal one with the same finalis. It lies a fourth lower: its ambitus runs from a fourth below to a fifth above the finalis. You recognise it by the prefix “hypo-”, which means “under”. The table sets the two side by side. The difference is less a different scale than a different register of the melody: the scale stays the same, the centre of gravity shifts.

Why it mattered

In Gregorian chant the assignment to a mode determines the sound and expression of a melody. The classification by finalis and ambitus helped to order and compare chants. For playing and listening today a rule of thumb is enough: if a melody rises from its final note, it is authentic; if it lies around it, it is plagal.

Common mistakes

• Determining the mode by the finalis alone: Only the ambitus shows whether it is authentic or plagal.
• Equating “authentic” with “genuine”: The term orders the range; it says nothing about value or age.
• Confusing it with the authentic cadence: That is a term of harmony, not a term for a mode.