The starting point of most beginning studies of classical and state-space control is with control of a linear, single-input–single-output, time-invariant plant. The tools of classical discrete-time linear control system design, which parallel the tools for continuous-time systems, are the z-transform, stability testing, root locus, and frequency response methods. As in the classical approach to designing analog controllers, one begins with simple digital controllers, increasing their complexity until the performance requirements are met. Extending classical single-input, single-output control system design methods to the design of complicated feedback structures involving many loops, each including a compensator, is not easy. A tracking system is one in which the plant outputs are controlled so that they become and remain nearly equal to externally applied reference inputs. Similar to continuous-time systems, the root locus plot of a discrete-time system consists of the loci of the poles of a transfer function as some parameter is varied.