musica ficta

In music of the Middle Ages, the theory of nondiatonic notes (those notes not normally present in a simple C major scale). B flat was the first nondiatonic note allowed, in order to avoid the tritone B-F. Other notes later became sharpened or flattened for similar reasons. Musica ficta also introduced nondiatonic notes in order to create a semitone between the leading note (seventh note of a scale) and tonic (first note of a scale). For example, when the tonic is G the leading note F must become F sharp in order to have the desired semitone.

Musica ficta was not always written using accidentals (sharp, flat, or natural signs), rather it required precise knowledge by the performer of correct theory and practice.