What is the name of a pipe fitting that has both internal threads?

The correct answer is a bushing, which is specifically designed to adapt or connect pipes of different diameters. A bushing has internal threads on both ends, allowing it to be screwed into two sections of pipe. This feature is crucial in plumbing and piping systems, as it facilitates the transition between different sizes of pipes without the need for additional fittings.

The other fittings like elbows, couplings, and unions serve different purposes. An elbow changes the direction of the pipe, a coupling connects two pipes of the same size, and a union allows for quick disconnection of two pipe ends. While all these fittings are essential in various piping applications, they do not have the same threaded structure as a bushing. This distinction makes the bushing unique in applications where size adaptation is necessary.