Which plumbing fitting unites two pipes typically through welding?

The plumbing fitting that unites two pipes typically through welding is a coupling. Couplings are designed to connect two straight pipes, allowing them to be joined securely to facilitate the flow of fluids. In terms of plumbing applications, couplings often provide a smooth transition between the ends of two pipes, making them essential for creating a continuous pipeline.

In contrast, unions are designed to allow for easy disassembly of pipes while still providing a tight seal, which is not the primary function of a coupling. Valves serve a different purpose altogether by controlling the flow of fluid within a piping system, rather than simply connecting pipes. Reducers are fittings used to connect pipes of differing diameters, adjusting the flow size rather than merely joining two standard pipes.

Thus, the identification of a coupling as the correct answer highlights its fundamental role in plumbing systems, where it ensures a secure and reliable connection is achieved through methods such as welding.