An extension nut is a kind of fastener nut that is used to join two male threads together, such as multiple pieces of threaded rod. Extension nut is a much broader name for coupling nuts, which connect two male threads of the same thread size, and reducer nuts, which connect two male threads of different sizes.
Clamping hex nuts are specialized nuts designed to firmly grip and secure components together, often in situations requiring frequent tightening and loosening. They are commonly used in clamping applications, such as with indexing plungers, cam action indexing plungers, and collet chucks, and are often found in conjunction with washers or spherical seatings to accommodate positional inaccuracies.