The paper deals with the Russian tautologies X est'X 'X is X' and X eto X 'X this is X'. We elaborate on the diachronic and synchronic analyses of the two constructions based on the data from the Russian National Corpus. The paper discusses a) milestones in the development of these two tautologies in Russian (taking into account the pattern X sut'X 'X are X' that has completely disappeared from the language); b) frequency of different syntactic phrases used as the repeated element X (based on the Russian National Corpus data); c) syntactic constraints on phrases in both constructions; d) semantic and pragmatic features of the two types of constructions. We argue that Xest' X most often appeals to different components of X's meaning (e. g. its connotations), while X eto X expresses the subject's identity to itself, being a tautology in the Wittgensteinian sense.