DEMARCATING AND DELIMITING THE UMMA A well-established tradition in Middle Eastern historiography proposes that the Iranian-Ottoman boundary was firmly established in 1639; it is one of the oldest boundaries of the world, and even before the nineteenth century was well-defined. This paper argues that the history of this polyglot borderland defies such a simplistic description. Even in the 19th century, it resembled many things, but a pacified and settled frontier it was not. It took more than seven decades long (1843-1914), intermittent work of Russian, British, Ottoman, and Iranian technical and diplomatic teams to survey and demarcate this frontier extending from the Persian Gulf to Armenia. The paper contends that from the first treaty of 1639 to 1914 sovereignty over this borderland and the communities inhabiting it variously shifted between the Shi'i dynasties ruling over Iran and the Sunni Ottoman Empire. Yet, in times of both war and peace, the borderland witnessed a relatively free flow of goods and ideas; as well as of armies, diseases, refugees, nomadic and semi-nomadic tribes, fugitives, pilgrims, and many others. However, the human and political geography of this hitherto highly porous and ill-defined frontier region dramatically changed with its transformation into a boundary region in the period between 1840-1914. Although there exist some geopolitics informed studies on the river boundary between Iran and Iraq, studies on the history of people inhabiting their common frontier and the frontier itself are nearly non-existent. Based on a vast array of the Ottoman and Iranian documents, chronicles, and travelogues this paper provides a history of the transformation of the Ottoman-Iranian frontier into boundary and discusses the notions of frontier/border in the Islamic world. It highlights the different and changing meanings of frontier/border and sovereignty in the vernaculars of the Ottoman and Iranian states. It also aims to show how these notions changed in relation to the modern concept of territoriality and institutions related to it. Moreover, questioning the notion of umma the paper analyzes the Shi'i-Sunni perceptions of the community of believers and examines how Ottoman and Iranian states built their notions of citizenship and differentiated their territories and subjects from one another.