A system is said to be a single-variable system if it has only one input and only one output. A system is said to be a multi variable system if it has more than one input or more than one output. An equation that describes the relation between the input and the output of a system is called the input—output relationship, also known as the external description or the input—output description, of the system. In developing this relationship, we assume that the knowledge of the internal structure of a system is unavailable to us. Instead, the only access to the system is by means of the input ports and the output ports. Under this assumption, a system may be considered as a "black box." We apply inputs to a black box, measure their corresponding outputs, and then try to abstract key properties of the system from these input—output pairs [12].