A coupling nut is a threaded

Screw thread

A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread...

 fastener

Fastener

A fastener is a hardware device that mechanically joins or affixes two or more objects together.Fasteners can also be used to close a container such as a bag, a box, or an envelope; or they may involve keeping together the sides of an opening of flexible material, attaching a lid to a container,...

 for joining two male threads, most commonly threaded rod

Threaded rod

A threaded rod, also known as a stud, is a relatively long rod that is threaded on both ends; the thread may extend along the complete length of the rod. They are designed to be used in tension...

. The outside of the fastener is usually a hex so a wrench can hold it. Variations include reducing coupling nuts, for joining two different size threads; sight hole coupling nuts, which have a sight hole for observing the amount of engagement; and coupling nuts with left-handed threads.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 fastener 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 size and profile are called reducing coupling nuts. An optional hole called the sight hole is provided. This provides visual verification that the threads are engaged.