An important and revealing idealized special case is to consider applying the second law to the scenario of an isolated system (called the total system or universe), made up of two parts: a sub-system of interest, and the sub-system's surroundings. These surroundings are imagined to be so large that they can be considered as an unlimited heat reservoir at temperature T R and pressure P R – so that no matter how much heat is transferred to (or from) the sub-system, the temperature of the surroundings will remain T R ; and no matter how much the volume of the sub-system expands (or contracts), the pressure of the surroundings will remain P R.