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 fourth below, C, remained the lower limit (Powers 2001c). 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 tenor 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 sixth above the finalis, and the tenor of plagal mode 4—Hypophrygian—is therefore also a fourth above the finali