Given a planar set S of arbitrary topology and a radius r, we show how to construct an r-tightening of S, which is a set whose boundary has a radius of curvature everywhere greater than or equal to r and which only differs from S in a morphologicallydefined tolerance zone we call the mortar. The mortar consists of the thin or highly curved parts of S, such as corners, gaps, and small, connected components, while the boundary of a tightening consists of minimum-length loops through the mortar. Tightenings are defined independently of shape representation, and it may be possible to find them using a variety of algorithms. We describe how to approximately compute tightenings for sets represented as binary images using constrained, level-set curvature flow.