bé carré ("square b"). In German music notation, the letter B or b always designates B while the letter H or h – a deformation of a square b – designates B. 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. Strictly speaking the medieval signs and indicated that the melody is progressing inside a (fictive) hexachord of which the signed note is the mi or the fa respectively. That means they refer to a group of notes around the marked note, rather than indicating that the note itself is necessarily an accidental. For example, when a semitone relationship is indicated between F and G, either by placing a mi-sign () on F or a fa-sign () on G, only the context can determine whether this means, in modern terms, F-G or F-G, or even F–G. The use of either the mi-sign on F or the fa-sign on G means only that "some kind of F goes to some kind of G, proceeding by a semitone". As polyphony became more complex, notes other than B required alteration to avoid undesirable harmonic or melodic intervals (especially the augmented fourth, or tritone, that music theory writers referred to as diabolus in musica, i.e., "the devil in music"). Nowadays "ficta" is used loosely to describe any such un-notated accidentals. The implied alterations can have more than one solution, but sometimes the intended pitches can be found in lute tablatures where a fret is specified. The convention of an accidental remaining in force through a measure developed only gradually over the 18th