A set screw, also known as a blind screw or a grub screw, is a type of screw generally used as a detent to secure an object within or against another object, usually without using a nut (see bolts compared with screws). The most common examples are securing a pulley or gear to a shaft. Set screws are usually headless, meaning that the screw is fully threaded and has no head projecting past the thread's major diameter. If a set screw does have a head, the thread will extend all the way to the head (whereas a bolt might have an unthreaded shank between the head and thread). A set screw is almost always driven with an internal-wrenching drive, such as a hex socket (Allen), star (Torx), square socket (Robertson), or a slot. The set screw passes through a threaded hole in the outer object and is tightened against the inner object to prevent it from moving relative to the outer object. It exerts compressional or clamping force through the bottom tip that projects through the hole.