A Socket cap screw, also known as a Socket screw or Allen bolt, is a screw with a hexagonal inner hole in the head. It can only be tightened or loosened after the hex key, Allen wrench or Allen key is inserted into the inner hole. Hexagon socket screws are most commonly used in cylindrical head screws, head diameter about 1.5 times the main diameter of thread (1960 series), Other head types include Bottom head cap screws, which give an aesthetically pleasing surface, and Countersunk head cap screws, which suit tapered screw holes. Countersunk hole design allows the screw head to rotate without being exposed to the surface of the fixed object, so it is often used in small surfaces where traditional spenders are inconvenient to use.