In music, any one of eight scalar modes employed for medieval liturgical melodies. The modal system was conceived for the purpose of codifying plainchant (see Gregorian chant); the names of the modes were borrowed from the system used by the ancient Greeks, though the Greek system was inadequately understood and the connection between the two is illusory. The modes are distinguished according to the note used as the final (last note) and the emphasis placed on another note, called the dominant. The Dorian mode's final is D, the Phrygian mode's is E, the Lydian mode's is F, and the Mixolydian's is G. Each of these four original modes had a parallel mode (Hypodorian, Hypophrygian, Hypolydian, and Hypomixolydian) with a lower range. Though they principally employ the tones A-B-C-D-E-F-G, some replace B with B-flat. In the 16th century, further modes were identified—the Aeolian, on A, and the Ionian, on C (corresponding to modern minor and major). The mode on B was ignored because of B's problematic tonal relationship within the scale.

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