A hex coupling nut is a threaded fastener for joining two male threads, most commonly a threaded rod but also pipes. The outside of the fastener is usually a hex so a wrench can hold it. These are used to make up long rod assemblies from shorter lengths of rods. The rods are threaded into the coupling nut for a certain distance. Coupling nut dimensions are described by the Industrial Fasteners Institute in standard IFI-128. It is given a plain finish. It is either cold drawn or hot rolled depending on the bar stock used. Coupling nuts threaded with two different thread sizes and profiles are called reducing coupling nuts and can be used to tighten a rod assembly inward or to press a rod assembly outward.
