What type of fitting is commonly used to change the direction of a pipe?

The commonly used fitting to change the direction of a pipe is an elbow fitting. Elbows are specifically designed to create a bend in the piping system, facilitating changes in direction at various angles, most often 45 or 90 degrees. This is crucial in plumbing systems, where pipes often need to navigate around obstacles or reroute to reach fixtures efficiently.

Couplings are used to connect two straight sections of pipe, while caps are used to close the end of a pipe. Unions provide a means to connect two pipes while allowing for easy disconnection without cutting the pipe. None of these fittings serve the primary purpose of directing the flow of fluid like an elbow does, which is why the elbow is the appropriate answer in this context.