Bolts use a wide variety of head designs, as do screws. Theseare designed to engage with the tool used to tighten them. Some bolt headsinstead lock the bolt in place, so that it does not move and a tool is onlyneeded for the nut end.
Common bolt heads include hex, slotted hex washer, and socketcap.
The first bolts had square heads, formed by forging. Theseare still found, although much more common today is the hexagonal head. Theseare held and turned by a spanner or wrench, of which there are many forms. Mostare held from the side, some from in-line with the bolt. Other bolts haveT-heads and slotted heads.
Many screws use a screwdriver head fitting, rather than anexternal wrench. Screwdrivers are applied in-line with the fastener, ratherthan from the side. These are smaller than most wrench heads and cannot usuallyapply the same amount of torque. It is sometimes assumed that screwdriver headsimply a screw and wrenches imply a bolt, although this is incorrect. Coachscrews are large square-headed screws with a tapered wood screw thread, used forattaching ironwork to timber.
Head designs that overlap both are the Allen or Torx heads;hexagonal or splined sockets. These modern designs span a large range of sizesand can carry a considerable torque.