Reducing couplings are used to connect pipes of dissimilar diameters in a straight line. A reducing coupling has two different sizes of threads on each side.

A merchant coupling is a fitting used to join pipes, of similar size, together in a straight-line run. Merchant steel fittings are designed with Dryseal Taper Threads (NPTF) for more critical applications. NPTF Dryseal threads produce a mechanical seal by the mating and slight crushing of the threads when a wrench is applied to tighten the fittings. Half Merchant Couplings are about half the length of Full Merchant Couplings.