Typological regularities in the history of phonetic system: about the fate of medium sounds in Proto-Slavic period

The article defines the importance of typological research in examining the history of the phonetic system. It describes such typological pattern as variability of every kind of medium sounds. In Slavic language history the phonetic processes began most commonly in the area of medium sounds, i.e. sonants between vowels and consonants. Intermediate sounds have a special historical role on different stages of development of Slavic phonetics.