A cartoon characterization of the asymmetry implicit in thermodynamic change from a constrained ( " ordered " ) state to a less constrained ( " disordered " ) state, which tends to occur spontaneously ( an orthograde process ), contrasted with the reversed direction of change, which does not tend to occur spontaneously ( a contragrade process ), and so only tends to occur in response to the imposition of highly constrained external work ( arrows in the image on the right ).
