Plagal mode

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A plagal mode (from Greek πλαγιος 'oblique, sideways') ^[1] is a musical mode, and one of four Gregorian modes whose tonic is the fourth note of the scale. These four modes correspond to the modern modal scales starting on A (Aeolian), B (Locrian), C (Ionian), and D (Dorian), though the correspondence is not exact since the tonic in plagal modes is not the first note of the scale. The other four Gregorian modes are the authentic modes.

The plagal mode was developed by Anicius Manlius Severinus Boethius (480-524), who discovered the concept of Greek musical modes almost by chance.

However, Hucbald (840-930) saw Boethius's modes and created a series of either modes, separated into two pairs: Authentic and Plagal modes. The plagal modes are the even modes, numbered 2, 4, 6 and 8. The mode always being a 4th below the Authentic mode, the tonic is always the same as the authentic mode, but the dominant is different. As a general rule, the dominant is 3 above the tonic, except in hypophrygian, which is a 4th above. This is because a 5th above the tonic of the Phyrigian mode is B, which was considered dissonant to the early Roman Church, as an accidental use of this note would produce a tritone.