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.