Screws are used to hold two objects together or to fasten one object to another. Some screws are designed to screw into a pre-threaded material (internal thread) such as a nut or a theaded hole while others screw directly into a surface while creating their own hole. Common screw head types include: round, flat, pan, oval, truss, and hex. Drive types include, phillips, slotted, torx (allen), hex and socket. The most popular screws are: wood screws, stainless steel screws and machine screws.