The commission of delegates from the bey in Tunis rode the frontier. Marking certain hills, certain wells, and the movements of regional tribes, they described the border to the French Marshal Jacque-Louis Randon. This work created a line for cartographers delineating the frontier between the two powers in 1846. That line’s functionality?its reality as a boundary?was the base of modern sovereign power along the frontier. But how to make that power a reality? Few people who lived in the region in 1846 believed in the border. For instance, border crossings in 1851 were of such a problematic nature that the Algerian and Tunisian authorities agreed another border commission was necessary. In 1861 tensions along the same frontier resulted in the French military deputizing local Algerians to police the border. In 1852 and 1856, 1865 and 1872, 1883 and 1916 we can find similar stories – all indicating the profound unreliability of the border as boundary. This indetermination of the border is a reflection of the indeterminate articulation of modernity in the colonial project. Far from a failure of modern sovereignty, this indeterminacy should be seen as a central component of it. These and other incidents suggest a reading of the border that puts the ongoing practice of “bordering” into conversation with other aspects of social change associated with the appearance of modernity in the region. The bounding of the world into discrete binaries?nature/culture, economy/society, modern/traditional, inside/outside?was the product of repeated performances of those boundaries. The Algerian-Tunisian border is one site where we can watch this performance and the reality it created. This paper will explore how the frontiers of modernity in Algeria depended on the repeated performance of the boundaries between interior and exterior. Based on bureaucratic correspondence, military reports, and colonial newspapers, this paper demonstrates that the limits of modernizing sovereignty in North Africa were produced and reproduced through the constant repetition of boundary performance.