ThisSystem chapterState of a system clarifies what a thermodynamic system is and how its state can be described. Every system is separated from an environment by a system boundary. This is done analogously to other technical disciplines, e.g. in technical mechanics. First, the permeability of the system boundary is categorised. After classifying the system and the system boundary, the next step is to quantify the internal state of a system. This leads to so-called state variables, e.g. pressure and temperature, which determine the state of a system. Our everyday experience shows that the state can be changed by external influences across the boundary