Musica ficta
Medieval practice of raising or lowering notes without notating it — this is how the leading tone entered modal writing.
What it means
Musica ficta — “invented music” — designates the practice, in performance, of raising or lowering notes that do not stand written as altered in the notation. The expression at the same time means these notes themselves: ones that lie outside the regulated stock of notes of the hexachords and were therefore not regularly provided for in the theory of that time. The older designation for it is musica falsa.
Musical use
Two rules led to such alterations. On the one hand, the cadence demanded a leading tone: in modes whose seventh degree lies a whole tone below the finalis, it was raised at the close. On the other hand, it held to avoid certain intervals — the tritone in the course of a melody and the diminished fifth between voices; here the lowering of a note helped. A common mnemonic rule of the time demanded that no augmented formation be allowed to arise above a long note.
That these alterations often remained unwritten has a practical reason: they counted as the self-evident application of known rules, not as part of the text. For today's editions a well-known problem arises from this, because the rules are not unambiguous in every case and different editors arrive at different solutions. It is usual to place suggestions as small accidentals above the notes, in order to distinguish them from the transmitted text.
Historically the practice is significant, because through it the leading tone entered modal music — and with it an essential step toward major-minor tonality was taken.
Which notes exactly should be raised or lowered is one of the most difficult questions of the performance practice of early music. Guidelines were the avoidance of the tritone and the diminished fifth, as well as the raising of the leading tone at cadences.
Performance practice
In scholarly editions, watch the small accidentals above the staff and clarify beforehand within the ensemble which ones you will execute — inconsistent leading tones are immediately audible in a modal setting. Check contested spots by the combined sonority, not only by your own line. And do not apply the raising across the board: outside the cadence it is usually not meant.