HEXACHORD. - A system, or musical scale, of six sounds adopted around the 11th century in musical teaching, together with the monosyllables ut re mi fa sol la.
The hexachord in its succession: tone-semitone-tone-tone, is reproduced three times in the diatonic musical diagram: rather than giving a different name to the two semitones of the seven-sound scale, the semitone interval was always called by the monosyllables mi-fa; then, to distinguish the hexachord beginning with ut from those beginning with fa and with sol, the hexachord of ut was called "naturalis"; the hexachord of fa with B-flat: hexachord "mollis", and that of sol with B-natural: "durum". It is understood that when theorists speak of "cantus naturalis", "cantus mollis", "cantus durus", these qualifications refer not to the character of the chant itself, but to the use of hexachords. Hexachordal solmization, if it was not inspired by the medieval repudiation of the tritone, favored its disappearance from monodic and polyphonic music after the 10th century. Hexachordal solmization, in use until the middle of the 18th century, influenced the evolution of the tonalities as they are known today: the principle of modulations in the modern sense lies in the practice of passing from one hexachord to another, each time a semitone different from the one sung previously is encountered; this passage, called "mutatio", "muance", is an innovation with respect to Hellenic and Greco-Roman music. In fact, from genus to genus, from tone to tone, from mode to mode, from tessitura to tessitura, from rhythm to rhythm, etc., they did not establish a process that allows the first note of the major triad always to be called by the same name; indeed in Greek music each of the modes had its own transposition scale and could not choose at will the absolute degree from which to begin.
Archaic-modern Gregorian modal theory believes it finds in the three hexachords "natural"-lower, "soft"-central, "hard"-upper, the melodic framework generating the most ancient Gregorian melodies, saying that the eight modes, authentic and plagal, are of late origin and do not explain the phenomena of Gregorian melodic construction.
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Pietro Thomas