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A plagal mode (from Greek πλάγιος 'oblique, sideways, athwart') ^[1] is a musical mode, which is one of four Gregorian modes whose range includes the octave from the fourth below the tonic, or 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.^[2]

The earliest definition of the term is found in Hucbald's treatise De harmonica (ca. 880), who specifies the range as running from the fourth below the final to the fifth above. Later writers extend this general rule to include the sixth above the final and the Fifth below, except for the Hypolydian mode, which would have a diminished fifth below the final and so the 4th below, C, remained the lower limit.^[3] In addition to the range, the tenor (cofinal, or dominant, corresponding to the "reciting tone" of the psalm tones) differs. In the plagal modes, the tenor is a third lower than the finalis of the corresponding authentic mode, except in mode 8 (hypomixolydian), where it is raised to a 4th above the finalis (a second below the tenor of the authentic mode, 7) in order to avoid the "unstable" degree B/B♭ (in the authentic mode 3, the tenor is similarly raised to the 6th above the finalis, and the tenor of plagal mode 4—hypophrygian—is therefore also a fourth above the finalis).

The term "plagalis" used in Western chant theory has a precise equivalent in Byzantine modal theory, where the word "plagios", refers to the four lower-lying echoi.^[4]

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