Bolts

A bolt is a form of threaded fastener with an external male thread requiring a matching pre-formed female thread such as a nut. Bolts are very closely related to screws

Bolts use a wide variety of head designs, as do screws. These are designed to engage with the tool used to tighten them. Some bolt heads instead lock the bolt in place, so that it does not move and a tool is only needed for the nut end.

Common bolt heads include hex, slotted hex washer, and socket cap.

The first bolts had square heads, formed by forging. These are still found, although much more common today is the hexagonal head. These are held and turned by a spanner or socket, of which there are many forms. Most are held from the side, some from in-line with the bolt. Other bolts have T-heads and slotted heads.