A carriage bolt, coach bolt or round head square neck bolt 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.