Summary and Keywords
Since roughly the late 19th century, international borders have generally been characterized by linearity, or the appearance as a series of one-dimensional points, connected by straight lines. Prior to this, various kinds of frontiers existed globally, some of them being more linear than others, but most included some kind of formal ambiguity. International relations (IR) often takes for granted the historical process which brought about the global linearization of borders, culminating in the late 19th century and still ongoing in ocean spaces and in outer space. But because cross-border relations are the main substance of inquiry in IR, many theories and areas of study in IR contain some perspective on that process, at least implicitly.
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