What fitting would be used to connect pipes and maintain the same diameter throughout?

The fitting that connects pipes while maintaining the same diameter throughout is a coupling. Couplings are designed specifically for this purpose; they join two sections of pipe in a straight line without changing the diameter of the pipe. This allows for a seamless connection that is essential when extending a pipe run or repairing sections of piping while ensuring fluid flow remains consistent.

On the other hand, a cap is used to close off the end of a pipe, a reducer is utilized when transitioning from a larger diameter pipe to a smaller one, and a 45° elbow changes the direction of the pipe while also not maintaining a uniform diameter. Each of these fittings serves a different function, emphasizing the unique role of a coupling in pipe connections.