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 dome like 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. These cap nuts are composed of a solid brass hex nut and top cap.