In Byzantine modal theory ( octoechos), the word "plagal" ("plagios") refers to the four lower-lying echoi, or modes. 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 ti, which may be flattened (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 finalis). In addition to the range, the tenor (cofinal, or dominant, corresponding to the " reciting tone" of the psalm tones) differs. 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. 880), who specifies the range as running from the fourth below the final to the fifth above. The earliest definition of plagal mode is found in Hucbald's treatise De harmonica (c. 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. Other Eastern Christian rites use similar systems of eight modes see Syriac usage of Octoechos and Armenian usage of Octoechos.Ī plagal mode (from Greek πλάγιος 'oblique, sideways, athwart') has a range that includes the octave from the fourth below the final to the fifth above. However, they are numbered differently, the authentic modes being 1, 2, 3, 4. ἦχος – echos), each consisting of a small family of closely related modes that, if rounded to their diatonic equivalents, would be the eight modes of Gregorian chant.
The older Byzantine system still retains eight echoi (sing.
This is because a fifth above the tonic of mode 3 is the "unstable" ti (in modern solfège), which may be flattened to ta. The tenor, or dominant (corresponding to the " reciting tone" of the psalm tones), is a fifth above the final of the scale, with the exception of mode 3 (Phrygian), where it is a sixth above the final. These four modes correspond to the modern modal scales starting on re ( Dorian), mi ( Phrygian), fa ( Lydian), and so ( Mixolydian). The range of mode 5 (Lydian) does not employ a subfinal, and so always maintains F as its lower limit. This added degree is called the "subfinal" which, since it lies a whole tone below the final, is also the "subtonium" of the mode. The final of an authentic mode is the tonic, though the range of modes 1, 2, and 7 may occasionally descend one step further. The authentic modes were the odd-numbered modes 1, 3, 5, 7, and this distinction was extended to the Aeolian and Ionian modes when they were added to the original eight Gregorian modes in 1547 by Glareanus in his Dodecachordon.
The eight Gregorian modes: f indicates 'final' Authentic mode
In the traditional system of eight modes (in use mainly between the 8th and 16th centuries) there are four pairs, each pair comprising an authentic mode and a plagal mode. This system likely originated from the early Byzantine oktōēchos, as indicated by the non-Hellenistic Greek names used in the earliest Western sources from about 800. By the late eighth century, a system of eight modal categories, for which there was no precedent in Ancient Greek theory, came to be associated with the repertory of Gregorian chant. The repertory of Western plainchant acquired its basic forms between the sixth and early ninth centuries, but there are neither theoretical sources nor notated music from this period. The theoretical framework of modes arose later to describe the tonal structure of this chant repertory, and is not necessarily applicable to the other European chant dialects ( Old Roman, Mozarabic, Ambrosian, etc.). The name of Pope Gregory I was attached to the variety of chant that was to become the dominant variety in medieval western and central Europe (the diocese of Milan was the sole significant exception) by the Frankish cantors reworking Roman ecclesiastical song during the Carolingian period.
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