Generally, countersunk heads are used in areas where smooth surfaces without protrusions are required after connecting bolts, as countersunk heads can be screwed into parts. Round heads can also be screwed into parts. The tightening force of the square head can be greater, but the size is quite large. Secondly, in order to meet the needs of locking after installation, there are holes in the head and rod, which can prevent the bolts from loosening when subjected to vibration. Some bolts without threads need to be made thin, called slim waist bolts. This type of bolt is beneficial for connecting under variable forces. There are specialized high-strength bolts on the steel structure, with larger heads and varying sizes. Then there is another special use: T-slot bolts, which are most commonly used on machine tool fixtures and have a special shape that requires cutting off both sides of the head. Anchor bolts, used for connecting and fixing machines to the ground, come in many shapes.