Musica ficta has long been the subject of in-depth discussions by musicologists, including its adoption and applications in the domain of plainchant melodies, the domain where musica ficta was termed coniuncta by late-medieval theorists. This paper addresses the fifteenth century treatises belonging to the Johannes Hollandrinus tradition, the treatises which add new elements to these discussions. On the one hand, these texts contain descriptions of little-known practices of singing chant melodies ‘semitonialiter’; on the other, they explicitly formulate the idea that the use of semitones exceeding the Guidonian gamut constitutes a musical phenomenon which integrates vocal (both plainchant and polyphony) and instrumental music. Convenient comparative source material can be found in two Prague organ treatises (ca 1430) which demonstrate the extent to which hexachordal thinking and the theory of coniuncta determined the use of semitones on the keyboard.
