Musica ficta, musica falsa
In the music of the 10th to 16th centuries -- the theory of chromatic or,
more properly, nondiatonic tones. At an early time the b-flat was
admitted in practice (Gregorian chant)
as well as in theory and was, therefore, frequently regarded as not part
of musica ficta. The introduction of the nondiatonic tones resulted
from melodic modifications or from transpositions of the church modes.
Musica ficta enabled performers to introduce flats or sharps into the modes
in order to avoid certain intervals, or to make intervallic adjustments
for the sake of "beauty."
In modern musicological writings, musica ficta denotes not so much the
theory of early chromaticism but the problems arising from the fact that
chromatic alterations rarely are indicated in musical sources prior to
1600. In the 12th and 13th centuries the tonal resources of polyphonic
music are so limited, the harmonies so clearly modal,
that very seldom are accidentals needed in addition to those indicated
in the original sources.