Cap nuts, or acorn nuts, are hex nuts with a domed top.
Cap Nut, #8-32, Brass, Brass, 10 PK Size: #8-32Domed top covers and protects threadsCreates a finished appearanceAlso known as an acorn nutExposed threads should not be longer than inside height of cap nutThread Type: Coarse, Thread Class: UNC-2BSpecification(s) Met: ASME B1..1 They cover the end of the bolt tip, which protects the threads and provides a finished appearance. Since protruding bolt tips can catch skin or clothing, use of cap nuts can also help to prevent injury.