Its strength is three times that of small screws, making it the strongest among general standard screws. It has a hexagonal hole in the head and is usually used without a nut. Due to its excellent assembly and breakage prevention properties, it is used for securing machine tools and automotive parts. Typically, its surface treatment is black oxide. It can be tightened with a hex wrench. Compared to hex bolts, the head size is smaller. Therefore, to save space, the head is often recessed into the spot.