In the early Middle Ages, a widespread musical tradition was based on the hexachord system defined by Guido of Arezzo. The basic system, called "musica recta", had three overlapping hexachords. Change from one hexachord to another was possible, called a "mutation". A major problem with the system was that mutation from one hexachord to another could introduce intervals like the tritone that musicians of the time considered undesirable. To avoid the dissonance, a practice called musica ficta arose from the late 12th century onward. This introduced modifications of the hexachord, so that "false" or "feigned" notes could be sung, partly to avoid dissonance. 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.