The sacred
music of the Roman Catholic Church, including Gregorian chant,
was based on four modes: four called authentic (Dorian,
Phrygian, Lydian, and Mixolydian), and four called plagal
(Hypodorian, Hypophrygian, Hypolydian, and Hypomixolydian).
This modal
system is different from that of the original Greek modes due to
some misunderstanding of the latter.
(From the book Examples
of Gregorian Chant and Works by Orlandus Lassus, Giobanni
Pierluigi Palestrina and Marc Antonio Ingegneri compiled by
Gustave Fredric Soderlund and published by the
Appleton-Century-Crofts Publishing Company in 1937.)
