3-1/2" EUE P110 Tubing Coupling Product Description Tubing & Casing Coupling Coupling is the joining of two short length pipes by the means of male and female threads. All types of couplings have internal threads which are known as female thread forms. These thread forms are usually machined so that they can be coupled with the external threads on pipes known as male thread forms. Couplings are strictly designed as per API specifications. There are two types of tubing couplings widely used in the pipeline coupling manufacturing industries namely: