This paper deals with a frequency domain technique for stability analysis of linear feedback systems with periodically time-varying parameters. The approximate method of signal balance used by Clarket al. (1064) is generalized to yield better results in the case of a wide variety of systems including some TVP-coupled systems. For systems with TVP coupling, a novel and simple method of ‘pivotal points’ is suggested for quickly sketching the stability boundaries making use of the known stability of subsystems. The results of perturbation analysis (Rao 1969) are found to support the pivotal paint method.