In this paper we consider the stability of feedback systems in the presence of combined plant and controller uncertainty. We show that if a controller C stabilizes a ball of plants about a nominal plant P, where distances are measured by the gap metric, then P stabilizes a ball of controllers about C of the same radius. This reveals a certain reciprocity between the role of plant and controller in a feedback system. We give a necessary and sufficient condition for stability under simultaneous perturbations of the plant and the controller. We also introduce a weighted gap metric and discuss its application to the problem of robust stabilization of feedback systems.