Hexachord

Six notes with a fixed pattern

A hexachord is a series of six notes. Its step pattern is always the same: whole tone, whole tone, semitone, whole tone, whole tone. Each note is assigned a syllable: ut, re, mi, fa, sol, la. The only semitone always lies between mi and fa. Anyone who sings mi–fa therefore sings a semitone, whatever the pitch.

In the glossary: Solmization · Tetrachord · Musica ficta

Three hexachords

The medieval pitch space was built from overlapping hexachords beginning on different notes. A distinction was made between the natural hexachord on C, the hard (durum) on G with B natural and the soft (molle) on F with B♭.

Because each hexachord spans only six notes, you had to change when you crossed this range. That is called mutation: a note is thought of in two hexachords at once, and the syllable name changes. The C in the natural hexachord is ut, but in the durum hexachord on G it is fa.

On the name of the system: the lowest note was designated by the Greek letter gamma, from which, via the syllable naming, the French “gamme” and the English “gamut” for scale arose.

Outside the order

Notes that were not foreseen in this order counted as “musica falsa” or “musica ficta”. The word “false” is not a value judgement here but marks the unregulated.

Distinctions

The hexachord is not a tetrachord. The tetrachord is a group of four notes within a fourth, with which ancient theory built scales. The hexachord spans six notes and was the basis of medieval solmization.