A few years later (about 370 A.D.) Ambrose, the Archbishop of Milan, strove to define this music more clearly, by fixing upon the modes that were to be allowed for these chants; for we must remember that all music was still based upon the Greek modes, the modern major and minor being as yet unknown.  In the course of time the ancient modes had become corrupted, and the modes that Ambrose took for his hymns were therefore different from those known in Greece under the same names.  His Dorian is what the ancients called Phrygian, [G:  d’ d’’] dominant, A; his Phrygian was the ancient Dorian, [G:  e’ e’’] dominant, C; his Lydian corresponded to the old Hypolydian, [G:  f’ f’’] dominant, C; and his Mixolydian to the old Hypophrygian, [G:  g’ g’’] dominant, D. These modes were accepted by the church and were called the Authentic modes.

Almost two centuries later, Gregory the Great added four more modes, which were called Plagal or side modes (from plagios—­oblique).  These were as follows: 

(Keynote)

Hypodorian, [G:  a (d’) a’ ] dominant, F.
Hypophrygian, [G:  c (e’) b’ ] dominant, A.
Hypolydian, [G:  c’ (f’) c’’] dominant, A.
Hypo-mixolydian, [G:  d’ (g’) d’’] dominant, C.

It is easy to see that these so-called new modes are simply new versions of the first four; although they are lowered a fourth beneath the authentic modes (hence the hypo), the keynote remains the same in each instance.  Still later two more modes were added to this list, the Ionic, [G:  c’ c’’] dominant, G, which corresponded to the ancient Greek Lydian; and the Aeolian, [G:  a’ a’’] dominant, E, which, strange to say, was the only one of these newer modes which corresponded to its Greek namesake.  Naturally these two newly admitted modes were also accompanied by their lower pitched attendant modes, the Hypoionic, [G:  g (c’) g’] dominant, E, and the Hypoaeolian, [G:  e’ (a’) e’’] dominant, C.

SUMMARY

    Mode.  Key.  Dominant.

Dorian.            D        A
Hypodorian.        D        F
Phrygian.          E        C
Hypophrygian.      E        A
Lydian.            F        C
Hypolydian.        F        A
Mixolydian.        G        D
Hypo-mixolydian.   G        C
Aeolian.           A        E
Hypoaeolian.       A        C
Ionian.            C        G
Hypoionian.        C        E

    Dominants

    [G:  a’ f’ c’ {a (a’)} c’ a d’ c’ e’ c’ g’ e’]

Now all these lower, or derived modes, Hypodorian, Hypophrygian, Hypolydian, etc., received the name Plagal modes, because there was but one tonic or keynote in the scale; consequently a melody starting on any degree of the scale would invariably return to the same tonic or keynote.  They differed from the authentic modes, inasmuch as in the latter a melody might end either on the upper or lower tonic or keynote.  Thus the melody itself was said to be either authentic or plagal, according to whether it had one or two tonics.  The theme of Schumann’s “Etudes symphoniques” is authentic, and the first variation is plagal.