Early philosophical accounts of explanation mistook the function of boundary conditions for that of contingent facts. I diagnose where this misunderstanding arose and establish that it persists. I disambiguate between uses of the term "boundary conditions" and argue that boundary conditions are explanatory via their roles as components of models. Using case studies from fluid mechanics and the physics of waves, I articulate four explanatory functions for boundary conditions in physics: specifying the scope of a model, enabling stable descriptions of phenomena in the model, generating descriptions of novel phenomena, and connecting models from differing theoretical backgrounds
