ABSTRACT Purpose/rationale Competitive rules, which have implications for player safety, fairness, and entertainment, change over time, yet institutional accounts offer only a high-level view of rule change. This study centers, and looks within, competitive rules to examine how they evolve in professional sport. Design/methodology/approach Using an inductive approach, we conducted a qualitative document analysis of NFL rule books published between 1980 and 2020. We developed and examined longitudinal trajectories of player conduct rules. Findings The analysis reveals rule components (i.e. actions, actors, times, spaces, sanctions) and patterns of change. Rules generally expanded in scope and length. We draw on communications from NFL leaders to connect the findings to institutional factors. Practical implications The findings offer a framework for rule managers as they iterate rules to make sport safer, fairer, and more entertaining. Research contribution This study sheds light on how competitive rules evolve at the level of component parts, providing a foundation for future institutional scholarship on the causes and consequences of rule change. Originality/value This research offers a microfoundations and longitudinal understanding of rule change in sport, with insights for academics and rule managers.