Say you are making a jewelry box with curved sides. Because both pieces are curved across their width, the ends are also curved. With no straight edges to register your square and bevel gauge against, how are you going to lay out and cut dovetails on those curved parts? These nesting blocks are the answer. With mating faces cut to match the profile of the workpiece, they nest together to hold it tightly while laying out, shaping the workpiece, and cutting dovetails.

From Fine Woodworking #229