ENThe article is devoted to the analysis of medieval onims identifying the Lithuanian-Rhutenian nobility in the Grand Duchy of Lithuania. They were testified in the documents of the treaty in Horodlo (Akty Horodelskie 1413. Dokumenty i studia, eds. J. Kiaupienė, L. Korczak, Vilnius–Cracow 2013). In the history of anthroponymy of this social class, the 15th century was marked by moving from one-name designation and gradual establishing of two-name system – first name and second denotation. Patronymic surnames, nicknames, place names performed the function of proto-surname. Anthroponyms under examination, formed in multiethnic state reflect multi-dimensional Baltic-Slavonic transpositions in phonetics and morphology. Key words: anthroponymy, onimic category, naming model, language transposition.