MPI-I-94-111

Let $S$ be a set of points in the plane. The width (resp.\ roundness) of $S$ is defined as the minimum width of any slab (resp.\ annulus) that contains all points of $S$. We give a new characterization of the width of a point set. Also, we give a {\em rigorous} proof of the fact that either the roundness of $S$ is equal to the width of $S$, or the center of the minimum-width annulus is a vertex of the closest-point Voronoi diagram of $S$, the furthest-point Voronoi diagram of $S$, or an intersection point of these two diagrams. This proof corrects the characterization of roundness used extensively in the literature.