Drawn for the most part from a familiarity with the relevant British primary sources, this chapter chronicles the vexed process of narrowing the PersoOttoman frontier during the mid-nineteenth century. Having established that these borderlands had long functioned as a traditional frontier or border march, it then scrutinizes the roles played by Britain and Russia (as the interventionary powers) and by the Ottoman Empire and Persia as the local sovereigns in these efforts to narrow a disputed and impermanent zone into a mappable line from 1843 to 1876. It will be argued that Britain’s rationale for stabilizing the frontier – casting in stone the territorial balance that they estimated to prevail along the borderlands in 1843 – was not only ﬂawed in principle but impossible, in practice, to establish on the ground on any convincing basis. Britain would demonstrate in all of this that its understanding of a socially and historically complex borderlands was rather superﬁcial. If this was one reason why the project remained unﬁnished in the mid-1870s, the main one was that Britain and Russia did not possess the authority to impose the settlement they sought. Nevertheless, one consequence of this imperial project was to progressively instil a more developed territorial consciousness or defensiveness in the local disputants, whereby territorial deﬁnition in itself would become a major component of identity and rivalry. Customary transboundary movements would increasingly be viewed by central authority in Constantinople and Tehran as unwanted violations of national territory. Essentially, it will be argued that the Ottoman Empire saw participating in efforts to settle the frontier question as a chance not only to familiarize itself with remote and inaccessible terrain but to actually materially improve its territorial deﬁnition at the expense of Persia. For its part, Persia was notably pragmatic throughout this episode, believing that it would secure a more favourable deﬁnition by supporting the efforts of the mediating powers than it would by confronting Constantinople directly.