Falsa Musica
Medieval designation for chromatic notes.
What it means
Musica falsa — literally “false music” — is a medieval designation for notes that lie outside the regulated tonal system, that is, outside the hexachords from which the music theory of that time built its pool of notes. “False” is not meant pejoratively here but technically: these notes do not belong to the regular order and cannot be solmized in it. Later the designation musica ficta, “feigned music,” prevailed for the same matter.
Musical use
Such notes became necessary for two reasons. First, cadence formation demanded a leading tone that in several modes was not part of the scale; it was produced by raising. Second, undesired intervals had to be avoided, above all the tritone in the melodic line and the diminished fifth between voices; for this a note was lowered. Both often happened without notation: the singers knew the rules and applied them, so that the manuscripts contain fewer accidentals than actually sounded.
For present-day editions a well-known problem results. Editors must decide where such alterations are to be made and usually put their suggestions as small accidentals above the notes to separate them from the transmitted text. The rules are not unambiguous in all cases, and different editions of the same piece arrive at different solutions.
Musica falsa is to be distinguished from the later chromaticism of major–minor tonality: there the chromatic note is part of a developed system, here a departure from a modal order.
The expression was by no means meant pejoratively in medieval theory. It designated notes outside the Guidonian hand that were nevertheless used in practice, and is largely synonymous with musica ficta. The theorists stressed that such notes were indeed false in the sense of the system but necessary for euphony.
Performance practice
If you sing from a scholarly edition, watch the small accidentals above the staff and settle in the ensemble which ones you execute — nothing disturbs a modal piece more than inconsistent leading tones. Check disputed places against the sonority: what produces a tritone between the voices is usually not meant.