Hexachord
Six-degree tonal space after Guido of Arezzo.
What it means
The hexachord is a series of six notes with the fixed step sequence whole–whole–half–whole–whole. The semitone thus always lies between the third and fourth notes — between the syllables mi and fa. On this unit rested the medieval doctrine of tones and with it solmization.
Musical use
The tonal space was built up through overlapping hexachords beginning on different notes. Since each comprises only six notes, a change was necessary when this space was exceeded — the mutation, in which a note is thought in two hexachords at once and the syllable naming is changed. The advantage of this system lies in the fact that the semitone always lies at the same place: whoever sings mi–fa sings a semitone, regardless of the pitch.
Notes outside this order were not provided for in the system; for them the designation musica falsa or musica ficta applied, and it is precisely from this situation that the word “false” is explained — not as a value judgment but as a marking of the unregulated.
From the lowest note of the system, designated with the Greek letter gamma, arose through the syllable naming the designation for the whole system and finally the French gamme and the English gamut for scale.
The hexachord is to be distinguished from the tetrachord, the four-note group within the frame of a fourth, with which ancient and later theory built scales.
The six syllables ut, re, mi, fa, sol, la run in every hexachord with the semitone in the middle between mi and fa. One distinguished the natural hexachord on C, the hard one on G with B natural, and the soft one on F with B♭. When the tonal space was exceeded, the singer changed by mutation into another hexachord, a note receiving a new syllable in the process.
Performance practice
If you work with early music or with solmization, impress on yourself the position of the semitone between mi and fa — the whole system is oriented on it. Sing melodies on syllables before you play them; the assignment of syllable and position of the semitone sharpens the ear for the key.