Die-cast metal case holds its own against rough handling. Entire blade coated with heavy-duty Mylar polyester film for unbeatable durability. Two-rivet, corrosion-resistant hook stands up to continuous use. Special Tru-Zero hook does double-duty as a pivot for drawing circles and arcs. Mylar is a registered trademark of DuPont Teijin Films for its polyester film. Only Dupont Teijin Films makes Mlyar polyester film.