According to the permeabilities of the walls of a system, transfers of energy and matter occur between it and its surroundings, which are assumed to be unchanging over time, until a state of thermodynamic equilibrium is attained. The only states considered in equilibrium thermodynamics are equilibrium states. Classical thermodynamics includes (a) equilibrium thermodynamics; (b) systems considered in terms of cyclic sequences of processes rather than of states of the system; such were historically important in the conceptual development of the subject. Systems considered in terms of continuously persisting processes described by steady flows are important in engineering.