The Czech-Slovak communicative and dialect continuum represents a specific case of mutual relations among closely related languages. From among the plethora of topics related to the Czech-Slovak linguistic border, the chapter discusses the following five: (1) the development of a political border between the (future) Czech and Slovak linguistic communities; (2) the Czech-Slovak dialect continuum; (3) the diachrony of relations between Czech and Slovak and their influence on dialect classification; (4) innovations resulting from the dialects contact and dialect-to-standard advergence; (5) Czech-Slovak language relations from the perspective of trans-border communication.