Which component is used for joining two pieces of pipe together?

The component used for joining two pieces of pipe together is the pipe coupling. A pipe coupling is specifically designed for this purpose and serves as a fitting that connects two pipes end-to-end, allowing for the continuation of a pipeline without any interruption. It can come in various sizes and materials to match the specifications of the pipes being connected.

In contrast, the other options serve different functions: a pipe cap is used to close off the end of a pipe, a pipe elbow changes the direction of the piping system, and a pipe reducer allows for a transition between pipes of different diameters. Thus, while they are all important components in plumbing and piping systems, a pipe coupling is the dedicated fitting for joining two pipes directly.