The acorn nut is a type of fastener which gets its name from its shape. 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. In addition the dome gives a more finished appearance. It is usually made of brass, steel, stainless steel (low carbon content) or nylon. It can also be chrome plated and given a mirror finish.
There are two types of acorn nuts. One is low, or the standard acorn nut. The other is the high acorn nut. The high acorn nut is wider and higher and will protect extra long studs. There are also self-locking acorn nuts that have distorted threads in the hex area to create a tight friction fit to prevent the nut from vibrating loose.
There are standards governing the manufacture of acorn nuts. One is Society of Automotive Engineers (SAE) Standard J483, High and Low Crown (Blind, Acorn) Hex Nuts. Another is Deutsches Institut für Normung (DIN) 1587, Hexagon Domed Cap Nuts.