What is a distinguishing feature of carriage bolts?

A distinguishing feature of carriage bolts is that they have a round head and a square shank. The round head provides a smooth finish, which is aesthetically pleasing and reduces the risk of injury from sharp edges. The square shank is designed to prevent the bolt from turning when it's being tightened, allowing for a secure fit, particularly in wood or when using a nut. This combination of a round head and square shank makes carriage bolts unique and suitable for various applications where a flush finish is desired.

The other options do not accurately capture the defining characteristics of carriage bolts. For instance, while the shank is often flat, it is the square shank that truly differentiates this type of bolt, making it distinct from other bolt designs. Carriage bolts are also versatile in their use, suitable not only for wood but also for other materials when paired with appropriate washers and nuts. Lastly, carriage bolts are not shorter than hex bolts. They can vary in length similar to other types of bolts, making their size not a defining characteristic.