A carriage bolt, coach bolt or round head square neck bolt is a form of bolt used to fasten metal to wood. 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. Carriage bolts were developed for use through iron strengthening plates on either side of a wooden beam. It is commonplace though to use them to bare timber, the squared section giving enough grip to prevent rotation.