Complete Encyclopaedia of Music/A/Authentic modes or tones
Authentic modes or tones. In the ancient church music, the four modes, introduced by St. Ambrose, in which the principal notes of the melodies are confined within the octave above the key note ; in distinction from the plagal modes or tones, introduced by Gregory the Great, in which the principal notes of the melodies are confined within the octave below the fifth of the key.