Heat transfer is practically reversible when it is driven by practically negligibly small temperature gradients. Work transfer is practically reversible when it occurs so slowly that there are no frictional effects within the system; frictional effects outside the system should also be zero if the process is to be reversible in the strict thermodynamic sense. For a particular reversible process in general, the work done reversibly on the system, W A → → B p a t h P 0 , r e v e r s i b l e , and the heat transferred reversibly to the system, Q A → → B p a t h P 0 , r e v e r s i b l e are not required to occur respectively adiabatically or adynamically, but they must belong to the same particular process defined by its particular reversible path, P 0 , through the space of thermodynamic states. Then the work and heat transfers can occur and be calculated simultaneously.