includes the octave from the fourth below the final to the fifth above. The plagal modes are the even-numbered modes 2, 4, 6 and 8, and each takes its name from the corresponding odd-numbered authentic mode with the addition of the prefix "hypo-": Hypodorian, Hypophrygian, Hypolydian, and Hypomixolydian.

The earliest definition of plagal mode is found in Hucbald's treatise