Acorn Nuts:

An acorn nuts, also known as a cap nut, is an internally threaded fastener with a dome-shaped head. It can cover the ends of externally threaded fasteners for protection and aesthetics. An acorn nut is typically made from one of the following materials: steel, stainless steel, brass, aluminum, or plastic. Used alone or as a cap for a hex or jam nut. Acorn cap nuts serve a decorative purpose by concealing the threaded end of a fastener or acting as a cover for protruding, perhaps sharp threads.

General:

Cap Nuts, sometimes called Acorn Nuts, are hex nuts with a finished dome top that covers the exposed end of the bolt. This provides a finished look to the threaded end. Cap Nuts help prevent stripping of the threads and a provide a decorative appearance as a finishing fastener. Try to pick a wrench that is the closest size to the bolt and try different combinations of coins to get a snug fit. If the wrench is too small you can take a long nut and a bolt, screw the nut onto the bolt and tighten the bolt around the nut you want to tighten or loosen.