A carriage bolt is a type of fastener that can be made from a number of different materials. Carriage bolt are used to hold together wood with wood, wood with metal and even metal with metal. Their particular shape allows the bolt to be self-locking when placed through a square hole, especially when dealing with metal. Alternatively, it can also be easily placed through a round hole in most kinds of wood, making it a versatile accessory. Carriage heads are large smooth and domed with no drive.