Marchetto of Padua (the beginning of 14th century) is the most influential music theorist in Italy after Guido of Arezzo. Marchetto’s modal theory (from the treatise “Lucidarium in arte musice plane”) was the most quoted part of his heritage. His modal theory is an attempt to comprehend theoretically the phenomenon of modal variability in Gregorian chants: he submitted the original doctrine on commixture of modes (tonus commixtus). The present article is devoted to consideration of Marchetto’s innovative concept of interruptiones. Interruptio — «intermediation», «interruption» — the term entered by Marchetto for designation of different ways of melodic filling of species of a consonance (a 5th or 4th). As an eхample Marchetto presented eight ways of melodic filling of the first species of the 5th — d–a. Depending on its melodic filling a species of a 5th or a 4th can (or, on the contrary, cannot) introduce in a chant the features of other modes. Marchetto’s doctrine on interruptiones is a kind of practical handbook for the most exact definition of modal structurein the Gregorian chant; it also reflects interaction between scale and formula aspects of mode.