In the linear theory of elasticity Clapeyron's theorem states that the potential energy of deformation of a body, which is in equilibrium under a given load, is equal to half the work done by the external forces computed assuming these forces had remained constant from the initial state to the final state. It is named after the French scientist Benoît Clapeyron. Another theorem, the theorem of three moments used in bridge engineering is also sometimes called Clapeyron's theorem.