Bolts are threaded fasteners that require a preformed female thread such as a nut to match their male threads. In general, screws and bolts have a close relationship. In the same way that screws have many different types of heads, they also come in a variety of head designs. A tool is used to tighten them, and these are designed to engage with that tool. It is possible to use a tool only for the nut end of bolt heads, which locks the bolt in place so it cannot move.
