Hooks are load-transmitting components that are most often used in conjunction with chains and other equipment. Products such as eye hooks, clevis hooks, grab hooks, shank hooks, swivel hooks, and S hooks can be used to connect lifting and rigging attachments. Most industrial hooks are forged from alloy steel, carbon steel, or stainless steel and then heat treated, quenched, and/or tempered. They are then painted, self-colored, or finished with yellow chromate or bright zinc. Painted hooks come in colors such as red, blue, yellow, brown, and orange. Galvanized hooks are treated with zinc to provide corrosion resistance. They have a silverfish appearance and are designed for outdoor or marine applications. Some hooks are embossed with information such as chain grade, fractional size, and country of origin for traceability purpose. For example, clevis hooks and grab hooks may be rated for Grade 63, Grade 70, or Grade 80 alloy chain. Fractional sizes for hooks are measured in English units such as inches (in) or metric units such as centimeters (cm). Common English sizes for hooks include: 1/4, 3/4, 3/8, 5/8, 5/16, and 1/2 inches. Common metric sizes for hooks vary by manufacturer, distributor, or supplier. Hook manufacturers indicate whether their products are designed for overhead lifting and other specialized applications. Hook suppliers also indicate whether or not the ultimate strength of a hook equals 4 times the working load limit of the chain, in accordance with National Association of Chain Manufacturers (NACM) specifications. Ultimate strength is the tensile strength, per unit of surface area, at which an eye hook, clevis hook, grab hook, shank hook, or S hook will fracture or continue to deform under a decreasing load. Working load limit is the maximum load that a component is designed to support. Hooks for military applications need to meet U.S. military specifications (MIL-SPEC) or naval standards such as P/N 61A101B10. Aircraft tie-down hooks must also be designed for their specific applications. As a rule, hook distributors indicate whether a product is not designed for a particular purpose. For example, alloy shank hooks may be designated as not intended for threading or swaging. They do, however, include a pre-drilled boss for the addition of a latch.