Wikipedia states the following definition of a closed, thermodynamical system: "In a closed system, no mass may be transferred in or out of the system boundaries. The system always contains the same amount of matter, but heat and work can be exchanged across the boundary of the system." QUESTION: Can a closed system consist of separate parts? I.e. consider two baloons of gass, as a n approximation one baloon can certainly be considered as a system. But what if we consider the two baloons together -- would that also be a closed system? In other words, is a closed system allowed to have more that one boundary?