This chapter examines contemporary border theories. What do borders divide? An important account holds that borders’ main function is to exclude and separate: borders divide peoples or geographical areas. However, critical border scholars have argued that borders do not in fact divide; they are not “lines in the sand.” Rather than separations, borders are zones and practices that extend within and beyond territorial limits: they are everywhere. This chapter rejects both these accounts, which rely on sovereignty and determine the unity of the state in contrast with the outside: aliens, foreign countries, and exceptional legal decisions. The chapter maintains that borders do not divide peoples, geographical spaces, or flows: insofar as they divide, they divide jurisdictions. This distinction matters theoretically, because unlike sovereign territories, jurisdictions make legal spaces through practice; they do not make claims about the people’s identity, territorial integrity, or the legitimacy of rule.