The class 10.9 steel cap screw has a phosphate and oil finish and an external hex drive, and meets Deutsches Institut fr Normung DIN 6921 specifications. Class 10.9 steel, composed of quenched and tempered medium carbon steel, is often used in applications where strength is the primary concern. The material is magnetic. The phosphate and oil coating provides corrosion resistance and lubricity. The external hex drive has six flat sides that a wrench can grip to turn the screw from the side, enabling use when there is no space above the head for a driver.

Cap screws and hex bolts are threaded fasteners with thick hex-shaped heads that can be used with threaded nuts or in areas with limited clearance above the tapped (threaded) hole. Cap screws have slightly different dimensions than hex bolts and have a washer face underneath their unflanged hex head to act as a large bearing surface against the mating material.