To study language contact in the Polish-East Slavic borderland, we employ extensive subdialect records from atlases, dictionaries, monographic studies, and various file collections. Significantly, however, all of the above lack historical information about the words they contain. Such data can be obtained by using local names and by taking into account all pan-Slavic references. Such comparisons justify the conclusion that historically many of the presented names extended far further westward than is indicated by typically used materials, mainly from the 20th century, though much less frequently from the second half of the 19th century. This sheds new light on the problem of whether the names in question are loan words, naturally older than had previously been thought, or rather relics of former regional convergence, covering the broad Polish-Russian language borderland, and constituting the Mazovian-Russian community.