In theoretical investigations, it is sometimes assumed that one of the two systems is the surroundings of the other. Then it is assumed that the work transferred is reversible within the surroundings, but in thermodynamics it is not assumed that the work transferred is reversible within the system. The assumption of reversibility in the surroundings has the consequence that the quantity of work transferred is well defined by macroscopic variables in the surroundings. Accordingly, the surroundings are sometimes said to have a reversible work reservoir.