Coupling reducer nuts, sometimes just referred to as reducer nuts, are very similar to traditional coupling nuts. They are threaded fasteners used to join two male threads, most commonly threaded rod. The outside of a coupling nut is typically a hex to be driven with a wrench. Coupling reducer nuts are also used to make up long rod assemblies from shorter lengths of rods. Where they differ is in the threading. Reducer nuts have different thread diameters on each side, allowing the connection of two different sized male threads. They are often used in threaded rod assemblies where each rod is of a different size, in which they tighten they rod assembly inward or press the rod assembly outward.