What type of nut is known for having a rounded, smooth surface?

A cap nut is designed with a rounded, smooth surface, which completely covers the end of a bolt or screw. This design serves both aesthetic and safety purposes, as it prevents exposure to the bolt threads and minimizes the risk of injury from sharp edges. Cap nuts can also protect the threads from damage and corrosion, prolonging the lifespan of the fastened assembly.

The other types of nuts differ significantly in shape and function. For instance, wing nuts feature two "wings" that allow for easy hand-tightening and are not smooth, while stop nuts have an additional lock feature to prevent loosening under vibration. Hex nuts, which have a six-sided shape, provide a secure grip but do not offer the smooth, rounded finish characteristic of cap nuts.