It is distinguished from other bolts by its shallow mushroom head with a solid square underneath prior to the length of the bolt being a threaded bar. 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 bolt to be installed with a spanner or wrench, working from one side. The head of a carriage bolt is usually a shallow dome. The squared section is usually 2mm larger than the diameter of the bolt 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.