The article is devoted to the analysis of metro-rhythmic difficulties that church singers of the 16th century could experience in connection with: a) the shift in the status of the most prevalent duration from minima to semiminima and the appearance of fuses (future quavers) in the musical text, which gave rise to what K. Sachs in the 20th century called “horror fusae”; b) with the complication of the task of correct reading the triple meter sections of the musical text, creating one and a half proportions to the principal two-part time, which caused not only the recording “in four” of instrumental works conceived “in three” (in tablature), but also the alteration of proportional sections in many choral works created at the turn of the 15th—16th centuries into a duple meter form. The focus is on the consideration of decisions taken by the performers themselves in order to facilitate the interpretation of the metric aspect of music associated with a change in the principles of tactus.