The need for measuring the distance between control systems is basic in control theory. For example, in robust control theory, one knows that the unstable system P to be stabilized is just an approximation of reality, and so one would really like the stabilizing controller to not only stabilize the nominal system P, but also all sufficiently close systems P' to P. The question of what one means by "closeness" of systems thus arises naturally. So one needs a metric on the set of unstable systems which is amenable to computation and it has good properties in the robust stabilization problem. Such a desirable metric was introduced by Glenn Vinnicombe in 1993, and is called the ν-metric. There essentially the ring R of stable transfer functions was taken to be the set of the rational functions without poles in the closed unit disk. The problem of what happens when R is some other ring of stable transfer functions was left open. In this talk, we address this issue, and give an extension of the ν-metric.