This article focuses on the Tawang and West Kameng districts of Arunachal Pradesh, northeast India, collectively known as Monyul. It was ruled by Tibet for three centuries before the 1914 McMahon Line boundary included it in India. Even after that, cross-border exchanges between Monyul and Tibet continued until the 1962 Sino-Indian war, following which border passages between the two were closed. Today, Monyul is a marginal region, geographically distant from centres of industry and education, and lacking in terms of infrastructure. This article traces Monyul's marginality not simply to the border war, but to spatial practices of the British colonial state, beginning with the mapping of the boundary in 1914. It shows how Monyul was constructed as a buffer, despite being within a delimited boundary, first, by excluding it from regular administration, and, secondly, by pushing back the older Tibetan administration, thereby, making it (what I call) a ‘zone of difference/indifference’. But the buffer project was subject to contestation, mostly from the Tibetan religious aristocracy, whose temporal hold over, and material interests in, Monyul were challenged by the latter's incorporation into colonial India.