The much-lamented gap between theory and practice in education cannot be filled by practical knowledge alone or by explanatory knowledge alone. Principled practical knowledge (PPK) is a type of knowledge that has characteristics of both practical know-how and scientific theory. Like basic scientific theory, PPK meets standards of explanatory coherence. However, its main function is not explanation or prediction but practical guidance. PPK grows out of efforts to solve practical problems, but it requires additional effort invested in producing knowledge that goes beyond what is required for the task at hand yet not so far beyond as to be unusable by practitioners. The Wright brothers' construction of PPK to address problems of flight control is used as a model for building such knowledge in the learning sciences. Design-based research in the learning sciences may motivate research into basic theoretical questions, but it is unlikely to contribute directly to answering them. Extending design-based research to the creation of PPK can, however, increase the generalizability of knowledge produced through design work and provide a ladder leading to sometimes radical design improvement.