Acorn nut is its common name and it is also called a crown hex nut, blind nut, cap nut or domed cap nut. It is a nut that has a domed top to cover the threads. This is to prevent contact with the external thread to protect the threads or protect other things the threads would rub against. A closed nut having a domelike cap over the back, protruding part of the engaged screw or bolt. Cap nuts, also referred to as acorn nuts are used to cover exposed bolt ends and add a finished appearance.