A carriage bolt (also called coach bolt and round-head square-neck bolt) is a type of bolt. It is also known as a cup head bolt in Australia and New Zealand. It is distinguished from other bolts by its shallow mushroom head and the fact that the cross-section of the shank, though circular for most of its length (as in other kinds of bolt), is square immediately beneath the head. This makes the bolt self-locking when it is placed through a square hole in a metal strap. This allows the fastener to be installed with only one wrench, working from the opposite end. The head of a carriage bolt is usually a shallow dome. The shank has no threads and its diameter equals the size of the square cross-section.