Which fitting is specifically designed to join two pieces of pipe without changing the direction?

The fitting designed to join two pieces of pipe without changing the direction is a coupling. Couplings are used to connect straight lengths of pipe together, ensuring a smooth, linear transition from one pipe to the next without any angular deviation. This characteristic makes couplings essential for extending pipe runs, whether in plumbing, drainage, or other piping systems, while maintaining a direct flow path.

Reducers, while also used for joining pipes, serve a different purpose; they connect pipes of different diameters and can involve a change in flow direction depending on the configuration. A 90° elbow is specifically designed to create a change in direction by turning the flow of the pipe, and a cap is meant for sealing the end of a pipe, not for joining two pieces together. Thus, the coupling is the clear choice for this particular application.