A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or a object that has the internal thread formed into it. Other screw threads are designed to cut a helical groove in a softer material as it is inserted. Their most common use is to hold objects together or locate objects.