A carriage bolt is a bolt designed to be tightened with only one tool. Carriage bolts are ideal when only one side of an assembly has clearance to turn a wrench. Carriage bolts are commonly used in snow plows to attach the cutting edge to the blade.
Unlike the hex cap screw we all normally call a bolt, carriage bolts do not have a hex on one end. Instead, carriage bolts have a dome-shaped head. Under the head is a square area. It is this square that makes it possible to tighten these bolts with only one tool.
By itself, a carriage bolt doesn't work as planned. The square area under the head is designed to fit in to a square hole in the workpiece. It is this square hole that restrains the bolt and keeps it from turning when the nut is tightened.
