Noun
A series of six notes, with a semitone between the third and fourth, the other intervals being whole tones.
Source: Webster's dictionaryAt 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. Source: Internet
The note B is not present in the third hexachord hexachordum naturale (the natural hexachord) C–D–E–F–G–A. Source: Internet
This introduced modifications of the hexachord, so that "false" or "feigned" notes could be sung, partly to avoid dissonance. Source: Internet