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.

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