The second part of the book moves from the origins of linear borders to their consequences for international politics. Because linear borders are distinguished from other kinds of frontiers by certain technical practices, it is the expertise involved in these practices which forms a central part of how the linearization of borders makes a difference. Chapter 5 charts the emergence of modern boundary studies, focusing in particular on the writings of colonial surveyor Thomas Holdich, academic geographer Ellen Semple, and Viceroy of India Lord Curzon. This subfield grew partly out of a meeting between political geography and the practice of colonial surveying in the late nineteenth century. I show how boundary studies was able to give new life, for a few crucial decades, to the otherwise questionable and politicized idea of ‘natural boundaries’ as a respectable scientific concept, and argue that colonial knowledge was key to its emergence.