The present article investigates the Bulgarian tautologies X si e X and X e X on the basis of the Bulgarian National Corpus. The main purpose of this paper is: 1) to examine the lexical features of the binominative tautologies (noun type); 2) to determine the correlation between the noun type and semantic interpretation of tautologies; 3) to determine the role of the modal participle si in the tautological pattern. In the course of the analysis we compared the Bulgarian nominal tautological patterns X si e X and X e X in terms of their lexical, semantic, grammatical, syntactic and discursive peculiarity . The paper argues that these tautological patterns are to be distinguished. We affirm that in the patterns such as X si e X the particle si assumes the role of epistemic modality marker, emphasizing the certitude or incontestability of the judgement, which is an essential feature of the tautology Х copula Х in different languages. Moreover, this pattern is characterised by considerably larger replicability based on a certain degree of idiomaticity. Conversely, the pattern X e X is rather a freely formed construction serving as a model for utterances of the taxonomic characterization type with identical lexical components.