The distribution of work, knowledge, and responsibilities between doctors and nurses is a longstanding object of interest for medical sociologists. Whereas the strategies through which nurses and doctors construct their professional boundary have been thoroughly examined, little is known about why the regulation of the medical-nursing boundary varies across care settings. In the article, I argue that this gap in knowledge can be attributed to insufficient examination of the 'negotiation context', namely the features of the social and organisational environment that directly affect doctor-nurse boundary negotiations. Adopting a negotiated order perspective, and drawing data from a hospital ethnography, the article describes the different ways of constructing the medical-nursing boundary ($separating$, $replacing$, and $intersecting$) which were observed in three different care settings (a neurology ward, a neurosurgical ward, and an intensive care unit). Constant comparison of the observed interactional patterns led to the identification of three factors that significantly affected the construction of the medical-nursing boundary, specifically: patients' state of awareness, the type of clinical approach adopted by nurses and doctors, and the level of acuity on the ward. The article advances our knowledge of the medical-nursing boundary by shedding light on its flexible and contextual nature and by adding further nuance to the boundary-blurring/boundary-reinforcing dichotomy. New features of the 'negotiation context' are identified that enable more convincing explanations of why the medical-nursing boundary varies across care settings. Finally, the study advances the negotiated order theory by offering a framework for considering the structural differences that shape local negotiations.