Authentic
Webster's Dictionary [1]
(1): (n.) Vested with all due formalities, and legally attested.
(2): (n.) Having as immediate relation to the tonic, in distinction from plagal, which has a correspondent relation to the dominant in the octave below the tonic.
(3): (n.) An original (book or document).
(4): (n.) Of approved authority; true; trustworthy; credible; as, an authentic writer; an authentic portrait; authentic information.
(5): (n.) Authoritative.
(6): (n.) Having a genuine original or authority, in opposition to that which is false, fictitious, counterfeit, or apocryphal; being what it purports to be; genuine; not of doubtful origin; real; as, an authentic paper or register.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature [2]
is a term applied to the sounds connecting the final (in Gregorian music) with its octave, or a melody in which they only are employed, in contradistinction to those connecting the fourth below the final with its octave, the fifth above it, which were called plagal (q.v.). 'In Ambrosian music authentic scales only were employed,:and of these only four: the Phrygian (D-d), Dorian (E-e), Hypolydian (F-f), and Hypophrygian (G-g) of the Greek system. The .Eolian (A-a) and the Ionian (C-c), subsequently added to the number of the Church scales (tones or modes), were subjected to the same classification. Authentic scales are characterized by the harmonic division (6: 4: 3) of their octaves; e.g. C- gg-c; the plagal by the arithmetical division (4:3: 2); e.g. G-C - g. Authentic melodies are thought to have generally greater dignity and strength than plagal. A good modern example of the former is the well-known German chorale Eine feste Buig ist unser Gott, and of the latter the Evening Hymn, attributed to Tallis; and it would: be difficult to find in pure melodic music better examples of the sublime and the beautiful. The relations of subject and answer in the modern tonal fugue (as when C-g are "answered" not by g-d, but by g-C) obviously grew out of the division of scales into authentic and plagal
