The codification of British nationality in the early-to-mid twentieth century, beginning with the 1914 British Nationality and Status of Aliens Act, had a profound impact upon putative Britons living on the peripheries of empire, where the loss or acquisition of British status mattered a great deal. Consular discussions of petitions for British nationality and protection in the Chinese treaty ports suggest that the parameters of British legal belonging, which required more careful definition against the backdrop of extraterritorial rights and protections, were worked out in detail in jurisdictional borderlands on the edges of empire. As correspondence between individual constituents and consuls grappling with an evolving legislative landscape demonstrates, nationality became increasingly meaningful in concrete ways in the lives of ordinary settlers and sojourners in the twentieth century. The formal exclusion from this legal category of specific groups of people living beyond the boundaries of British territory, particularly ‘illegitimate’ children, married women, and children born to British mothers, and Straits Chinese individuals, had sharply felt effects upon the mobility, personal freedoms, and family cohesion of scores of settlers. The consequences of deprivation of the opportunities and protections of British status in China sharpened as the twentieth century progressed, bringing with it war, displacement, and regime changes.