hexachord
Noun
Meaning
A series of six notes, with a semitone between the third and fourth, the other intervals being whole tones.
Source: Webster's dictionarySynonyms
Examples
At first only B could be flattened, moving from the hexachordum durum (the hard hexachord) G–A–B–C–D–E where B is natural, to the hexachordum molle (the soft hexachord) F–G–A–B –C–D where it is flat.
The note B is not present in the third hexachord hexachordum naturale (the natural hexachord) C–D–E–F–G–A.
This introduced modifications of the hexachord, so that "false" or "feigned" notes could be sung, partly to avoid dissonance.