Double Normals and Tangent Normals for Polygons
Benjamin S. Halpern · Proceedings of the American Mathematical Society · 1975
Given a polygonal closed plane curve $\gamma$. Each segment of $\gamma$ has a tangent direction and a normal direction; each vertex of $\gamma$ has a cone of tangent directions and a cone of normal directions. Formulas are established connecting the numbers of various kinds of straight lines which either intersect $\gamma$ twice in a normal direction, or once in a normal direction and once in a tangent direction.