An acorn nut (sometimes called a cap nut) has a closed, domed top. This design covers the exposed threads of the bolt or stud. In automotive applications, they are commonly used as lug nuts on wheels. They protect the wheel studs from road salt, water, and road grime, preventing the threads from rusting solid.
Covering the threads also makes wheel removal easier. Without the acorn cap, road grime would pack into the exposed threads, making the lug nuts hard to spin off. Cap nuts ensure the threads remain clean and dry inside the dome, even after years of winter driving.
When installing acorn nuts, you must check the thread depth. If the bolt stud is too long, it will hit the inside of the dome before the nut clamps the wheel. This creates a dangerous situation where the nut feels tight but is not actually holding the wheel to the hub. Always verify that the stud length matches the nut's internal depth.