A plagal cadence is one in which the tonic chord is preceded by the sub-dominant chord (IV—I).
The plagal Cadence (A-men chord) at the end of the piece has been transcribed as breves instead of semibreves for authenticity.
These modes are divided into two classes—the “authentic” and “plagal.”
The theme of Schumann's “Etudes symphoniques” is authentic, and the first variation is plagal.
These enharmonic passages recur to satiety, and the abuse of the plagal cadence deprives it of its religious solemnity.
Thus the melody itself was said to be either authentic or plagal, according to whether it had one or two tonics.