This paper examines how a cognitive boundary with no physical presence has affected life in the cities of Jaffa and Tel Aviv, not only during its time of existence (1921-1950) but many decades after it was erased from all official documents. In 1921, the national aspirations of Jews in Jaffa, embraced by the local British Mandate government, triggered a segregation process that resulted in an official administrative split of Jaffa’s urban area and the creation of the “Hebrew” city of Tel Aviv on Jaffa’s northern parts. This administrative division had a clear ethnic character, dividing the entire urban fabric into a clearly defined “Jewish” and “Arab” geographical entities and influencing the development of the two municipalities as well as the daily life of their populations. After the 1948 War in Palestine, which led to the flight of almost all of Jaffa’s Arab population and the annexation its area to Tel Aviv, the united city continued to resemble a split city, with the former areas of Jaffa remaining relatively underdeveloped and neglected for decades. By combining spatial analysis and historical research, this study reveals how the “paper boundary” that was drawn between Jaffa and Tel Aviv in 1921 transformed the life of Arabs and Jews in the two cities in a way that undermined the physical unity of the urban fabric and the spatial potential of its street network. The creation of the municipal border led to the cognitive marginalization of the spatially central Manshiya neighbourhood, and later to its deterioration and eventual destruction. Ironically, the destruction of Manshiya gave a belated physical expression to the historic cognitive separation between the centres of Jaffa and Tel Aviv, working against the wish to unite the two cities into a single urban entity after 1948.@en