ESACORDO – The natural, soft, and hard hexachords are indicated in the top row. Along the vertical margin are marked the letters of the alphabet corresponding to the musical diagram. Horizontally are written two or three monosyllables (e.g., G, sol, fa, ut) which each note receives in solmization. Paris, Bibliothèque Nationale, ms. Lat. 1528, fol. 124r (14th cent.).
ESACORDO – A system or musical scale of six tones adopted around the 11th century in musical instruction, together with the monosyllables ut, re, mi, fa, sol, la.
The hexachord, 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 is always called by the monosyllables mi–fa. To distinguish the hexachord that begins on ut from those that begin on fa or sol, the ut hexachord was called “naturalis”; the fa hexachord with B-flat was called “mollis,” and the sol hexachord with B-natural was called “durum.” It is understood that when theorists speak of “cantus naturalis,” “cantus mollis,” or “cantus durus,” these qualifications refer not to the character of the chant itself but to the use of the hexachords.
Hexachordal solmization, if it did not inspire the medieval aversion to the tritone, at least contributed to its disappearance from monophonic and polyphonic music after the 10th century. Hexachordal solmization, in use until the mid-18th century, influenced the evolution of the tonalities as we know them today: the principle of modulations in the modern sense lies in the practice of moving from one hexachord to another whenever a different sentiment is encountered than that previously sung. This transition, called “mutatio” or “mutare,” represents an innovation with respect to Hellenic and Greco-Roman music. Indeed, from genus to genus, from tone to tone, from mode to mode, from tessitura to tessitura, from rhythm to rhythm, etc., no process was established that would allow the same name always to be given to the first note of the scale; rather, in Greek music each mode had its own transpositional scale and could not choose the absolute degree from which to begin.
The archaic-modern Gregorian modal theory believes it can find in the three hexachords—“naturalis” (lower), “mollis” (middle), and “durum” (upper)—the generative melodic framework of the oldest Gregorian melodies, asserting that the eight authentic and plagal modes are of later origin and do not explain the phenomena of Gregorian melodic construction.