Plagal mode

A Gregorian mode (or church mode) is one of the eight systems of pitch organization used in Gregorian chant.

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 (McKinnon 2001). 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 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. 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. This system likely originated from the medieval Byzantine oktōēchos, as indicated by the non-Hellenistic Greek names used in the earliest Western sources from about 800 (Powers 2001b, §II.1(ii)). Ignorant of these developments, Hucbald (840–930) created a series of eight modes (Powers 2001a).

In the traditional system of eight modes (in use from the 8th century up to 1547) there are four pairs, each pair comprising an authentic mode and a plagal mode.

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 (Powers 2001a). An authentic mode has its final as the lowest note of the scale, though in modes 1, 3, and 7 it may occasionally descend one note further, in which case this added scale degree is called the "subfinal" which, since it lies a whole tone below the final is also the "subtonium" of the mode. The range of mode 5 (Lydian) does not employ a subfinal, and so always maintains the note F as its lower limit (Powers 2001d). These four modes correspond to the modern modal scales starting on D (Dorian), E (Phrygian), F (Ionian = the Gregorian Lydian), and G (Mixolydian). 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. This is because a fifth above the tonic of mode 3 is the "unstable" B♮/B♭.

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