Abstract: One of the tasks of Slavic historical linguistics is to establish the relative chronology of the linguistic processes. For the Proto-Slavic language, this is practically the only way to identify the history of phonetic changes. It can be concluded that the qualitative differentiation of long and short vowels preceded the monophthongization of diphthongs, since otherwise the vowel [u] from *ou̯ would have coincided with *ū. There are, however, phonetic phenomena that date back to the written era, but due to the lack of reflection of the compared phenomena or the late reflection of one of them in the Old Russian writing, only a relative chronology can be established for them. This method determines that the closed [ô] of the Great Russian and South Ukrainian types arose in the early Old Russian period before the transformation of reduced vowels into full ones. The same method is used to chronologize some phonetic phenomena of the period of the separate existence of the East Slavic languages. For example, it is established that the change [ʧ’] into [ʃ’] in the western South Russian dialects occurred after the hardening of the old [ʃ’]; the hardening of consonants before [e] and [i] in Ukrainian is not an ancient process, since it took place after the secondary softening of consonants. And, finally, if we turn to morphology, we can state that the coincidence of the Nom. and Acc. Pl. in nouns denoting male persons occurred before the development of the category of animacy of these nouns. Thus, relative chronology turns out to be a necessary method not only in reconstructing the Proto-Slavic phonetic system in its dynamics, but also in determining the order of certain linguistic processes in the written period of the history of the East Slavic languages.