High Quality Content by WIKIPEDIA articles! In control theory, the root locus is the locus of the roots of the characteristic equation of the closed loop transfer function as the loop gain of the feedback system is increased from zero to infinty. Traditionally the root locus is constructed with the loop gain K as the variable. However, any system parameter can be used to construct a root locus. The root locus is a useful tool for analyzing the transient response, as well as the stability of a single input single output dynamic systems. A system is stable if all of its poles are in the left-hand side of the s-plane or inside the unit circle of the z-plane.