Acorn nuts are hex nuts with a dome top. Consequently, acorn nuts are also known as: cap nuts, crown nuts, dome nuts, hex cap nuts.
Used alone or as a cap for a hex or jam nut, acorn nuts serve a decorative purpose by concealing the threaded end of a fastener or acting as a cover for protruding, perhaps sharp threads.
Some types are manufactured in two pieces and pressed together, others are milled from a single piece of metal.
Aside from a possible slight difference in dimensions, both types of cap nut otherwise the same.