An example of a quasi-static process that is not reversible is a compression against a system with a piston subject to friction; although the system is always in thermal equilibrium, the friction ensures the generation of dissipative entropy, which directly goes against the definition of reversible. Alternatively, one can say that friction would generate heat and dissipative entropy only if the movement of the piston were not infinitely slow. A notable example of a process that is not even quasi-static is the slow heat exchange between two bodies on two finitely different temperatures, where the heat exchange rate is controlled by an approximately adiabatic partition between the two bodies— in this case, no matter how slowly the process takes place, the states of the composite system consisting of the two bodies is far from equilibrium, since thermal equilibrium for this composite system requires that the two bodies be at the same temperature.