Authentic mode
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An authentic mode is one of four Gregorian mode
Gregorian mode
A Gregorian mode is one of the eight systems of pitch organization used to describe 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 by the Frankish cantors reworking Roman ecclesiastical...

s whose final (or "tonic") is the lowest note of the scale (apart from the possibility of one "added note" below). Given the natural diatonic scale/mode of C Major, these four authentic modes are the ones that start on D (Dorian), E (Phrygian
Phrygian mode
The Phrygian mode can refer to three different musical modes: the ancient Greek tonos or harmonia sometimes called Phrygian, formed on a particular set octave species or scales; the Medieval Phrygian mode, and the modern conception of the Phrygian mode as a diatonic scale, based on the latter...

), F (Lydian
Lydian mode
The Lydian musical scale is a rising pattern of pitches comprising three whole tones, a semitone, two more whole tones, and a final semitone. This sequence of pitches roughly describes the fifth of the eight Gregorian modes, known as Mode V or the authentic mode on F, theoretically using B but in...

), and G (Mixolydian).

The other four Gregorian modes are the plagal mode
Plagal mode
A Plagal mode may mean different church chanting modes, depending on the context.-In Western Practice:A plagal mode   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...

s, and differ from the authentic modes in their range and reciting tones.

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
Octoechos
Oktōēchos is the name of the eight mode system used for the composition of religious chant in Syrian, Coptic, Byzantine, Armenian, Latin and Slavic churches since the middle ages...

, as indicated by the non-Hellenistic Greek names used in the earliest Western sources from about 800 (Powers 2001, §II.1(ii)). Ignorant of these developments, Hucbald
Hucbald
Hucbald was a Frankish music theorist, composer, teacher, writer, hagiographer, and Benedictine monk...

 (840-930) created a series of 8 modes, separated into two pairs: Authentic and Plagal modes. The authentic modes were the odd-numbered modes, 1,3,5,7. The tenor, or dominant (corresponding to the "reciting tone
Reciting tone
In chant, a reciting tone is a repeated musical pitch around which the other pitches of the chant gravitate, or by extension, the entire melodic formula that centers on one or two such pitches. In Gregorian chant, reciting tones are used for a number of contexts, including the chanting of psalm...

" of the psalm tones), is a 5th above the final of the scale, with the exception of mode 3 (Phyrigian), where it is a 6th above the final. This is because a 5th above the tonic of mode 3 is the "unstable" B/B♭.

For more information, see musical mode
Musical mode
In the theory of Western music since the ninth century, mode generally refers to a type of scale. This usage, still the most common in recent years, reflects a tradition dating to the middle ages, itself inspired by the theory of ancient Greek music.The word encompasses several additional...

.
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