HEXACHORD, in music, the scale of six degrees attributed to Guido d'Arezzo (I 1 th century) and introduced as an improve ment on the Greek scale system based on tetrachords or groups of four notes. The six notes making up each hexachord received the names ut, re, mi, fa, sol, la from the initial syllables of the lines of a certain hymn to St. John, the melody of which happened to be such that each line began on successive degrees of the hexachordal scale. As, however, the hexachords could begin on either C, F or G the syllabic names of the notes associated with the alphabetical names were not always the same. Thus in the scale of C the note G was sol, or G sol, whereas in the scale of F it was re and was therefore called G re, while in the scale of G itself it became naturally G ut. Hence in the complete gamut (a word derived from the note just named, gamma-ut) each alpha betical note was given a compound name according to the different syllabic names which coincided with it, e.g., G sol re ut, A la mi re, and so on. These names remained in use for cen turies until the whole system was superseded with the adoption of the octave as the standard group unit and recognition of the modern principles of key-relationship. But the same syllables ut, re, mi, etc.—are still retained, with the addition of si for the seventh note, in the musical nomenclature of France and Italy as the names for the notes of the existing diatonic scale of C major.
See GUIDO OF MEZZO ; MUSICAL NOTATION.