ESACORDO. – System ■ a musical scale of six notes adopted around the 11th century in musical instruction, together with the monosyllables ut re mi fa sol la.
The e., in its succession of 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-note scale, the semitone interval was always called by the monosyllables mi-fa; in order to distinguish the e. beginning with ut from those beginning with fa and sol, the e. of ut was called «naturalis»; the e. of fa with B-flat: e. «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 singing in itself, but to the use of the e. Hexachordal solmization, if it was not inspired by the medieval aversion to the tritone, nevertheless contributed to its disappearance from monodic and polyphonic music after the 10th century. Hexachordal solmization, in use until the middle of the 18th century, influenced the development of the tonalities as they are known today: the principle of modulations in the modern sense lies in the practice of passing from one e. to another whenever a semitone different from that previously sung is encountered; this passage, called «mutatio», «muance», is an innovation with regard to Hellenic and Greco-Roman music. Indeed, from one genus to another, from one tone to another, from one mode to another, from one tessitura to another, from one rhythm to another, etc., no process was established that would make it possible always to call the first note of the major triad by the same name; indeed, in Greek music each of the modes had its own transposition scale and could not freely choose the absolute degree from which to begin.
The archaic-modern Gregorian modal theory believes
that it finds in the three e.—the «natural»-lower, the «soft»-central, and the «hard»-upper—the melodic framework generating the most ancient Gregorian melodies, maintaining that the eight authentic and plagal modes are of late origin and do not explain the phenomena of Gregorian melodic construction.