Abstract: “The energy method for displacement calculation” stands out as a unique chapter in the material mechanics curriculum. In teaching practices, chanllenges arise from the need to navigate concepts such as dual variables (and space), external potential energy, and the principle of virtual stress. Additionally, since no textbook explicitly establishes the Principle of Virtual Displacement as a fundamental principle, the logical coherence within its content are not clearly articulated. To address this, this paper summarizes and discusses the interrelations among the Castigliano Theorem, Complementary Energy Theorem, Reciprocal Work Theorem, Reciprocal Displacement Theorem, Principle of Virtual Displacement, Unit Load Method, Graph Multiplication Method, and Regularization Equation of the Force Method. By clarifying the hierarchical progression of these concepts, the study delineartes a structured framework of the energy method's knowledge system.