Countersunk bolts are bolts designed to fit inside a countersunk hole. Countersunk bolts are mostly hidden when screwed in, because their heads are shaped to fit within the special holes. When these holes are empty, they look like a normal screw hole with a conical opening at the top; when they are full, the only visible part should be the very top of the screw or bolt. This conical opening allows countersunk bolts to screw into the hole and down into the opening, so the bolt head is flush with the surface of the object.
Countersunk bolts are used when a smooth surface is required. Common applications include: bridge decking, walkways, and railing.