ENThe author points out the consequences of the lack of demarcation between the Crown of the Polish Kingdom and the Grand Duchy of Lithuania in the 16th–18th centuries. She discusses the peculiarities of the Podlasie Province in terms of the status of its inhabitants (settlement by petty gentry, undefined legal status of such social groups as dependent gentry, boyars, landowners), the linguistic situation (bilingualism), the feeling of threat by soldiers, and above all the parallel functioning of two legal systems: Crown and Lithuanian in the post-Union period, using the example of the estate of the Radziwiłłs from Birżańsk, which included properties in both Crown and Lithuanian Podlasie. In conclusion, she refers to the relationship between the present day and historical notion of borderland, positing that in the case of Podlasie its mediating functions in terms of population flow and legal and civilisational influence outweighed the function of separating your own people from strangers, characteristic of the modern understanding of the term after the establishment of stable state borders.