Authentic mode
A church mode whose range extends an octave upward from the final.
What it means
In medieval modal theory, an authentic mode is one of the two basic forms every church mode can take. It is characterized by an ambitus, that is, the pitch range used, extending an octave upward from the final, the modal tonal center.
The eight classical church modes were arranged in the Middle Ages into four pairs, each sharing the same final: one authentic and one corresponding plagal mode. The Dorian (authentic) and Hypodorian (plagal) modes, for instance, form such a pair sharing the final D.
Musical use
The distinction between authentic and plagal served mainly to classify Gregorian melodies: by examining the ambitus and the typical melodic motion around the final (the reciting tone), one could determine whether a melody belonged to the authentic or the plagal form of a mode, even though both share the same final and the same pitch content.
The authentic mode should be distinguished from the plagal mode, which shares the same final but has an ambitus set a fourth lower, typically reaching from a fourth below to a fifth above the final. In notation and liturgical practice, authentic and plagal modes were often numbered using Latin ordinals (protus, deuterus, tritus, tetrardus, each in an authentic and a plagal form).
Performance practice
When analyzing Gregorian melodies, first determine the final and then the ambitus of the melody: if it extends mostly upward from the final, you are dealing with an authentic mode; if it reaches noticeably below the final, the plagal form applies.