<P> A carriage bolt, coach bolt or round head square neck bolt is a form of bolt used to fasten metal to wood . </P> <P> It is distinguished from other bolts by its shallow mushroom head and that the shank cross-section of the bolt is circular for most of its length, as usual, but the portion immediately beneath the head is formed into a square section . This makes the bolt self - locking when placed through a square hole in a metal strap, or a round hole in most wood . This allows the fastener to be installed with only a single tool, a spanner or wrench, working from one side . The head of a carriage bolt is usually a shallow dome . The squared section is of the same size as the diameter of the bolt shank, with a plain unthreaded shank . </P>

What is the purpose of a carriage bolt