Introduction and background: The roles of head nurse (HN) and charge nurse (CN) in hospital care units are intended to be complementary, but still remain ambiguous. Studying the complementarity of these roles is a unique way of looking at them. Objective: To explain in which circumstances the complementarity of HNs and CNs occurs in a hospital setting. Methods: A single case study with the inclusion of complementary data from fifteen interviews with HNs and CNs and from documentary sources (n = 80). Results: Fifteen independent or interdependent role components with potential complementarity were identified. This complementarity can be explained by way of three elements: 1) the CN’s partial perspective and the HN’s global perspective; 2) bidirectional communication; 3) common objectives requiring team mobilization. Discussion and conclusion: Studying complementarity is an innovative approach that provides an understanding of the work of both HNs and CNs, as well as allowing us to understand how they complement each other and how one’s work increases the value of the other’s and vice-versa, resulting in improved quality of care.