A nut is a type of fastener with a threaded hole. Nuts are almost always used in conjunction with a mating bolt to fasten multiple parts together. An acorn nut, also called a crown hex nut, blind nut, cap nut, domed cap nut, or dome nut, is a nut that has a domed end on one side. When used together with a threaded fastener, the domed end encloses the external thread to protect the thread or to protect nearby objects from contact with the thread.