The Phillips flat/countersunk head machine screws are bolts designed to fit inside a countersunk hole. A countersink is both the recessed hole and the tool to make the hole that allows compatible screws and bolts to screw in flush to a surface. Flat/Countersunk head screwss 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.The typical angle of a countersunk screw head is 90 degrees.
Screws Length= Head Height + Thread Length.