Although Carathéodory himself did not state such a definition, following his work it is customary in theoretical studies to define heat, Q, to the body from its surroundings, in the combined process of change to state Y from the state O, as the change in internal energy, Δ U Y, minus the amount of work, W, done by the body on its surrounds by the adiabatic process, so that Q = Δ U Y − W.