An elbow that has an outside thread on one end and an inside thread on the other is called what?

A fitting that has an outside thread on one end and an inside thread on the other is known as a street elbow. This particular configuration allows for joining two pieces of pipe together, where one piece would fit inside the fitting while the other would thread into the outside of the fitting.

Street elbows are particularly useful when there is a need to change the direction of the pipe without additional fittings, thus saving space and reducing the potential for leaks. This type of fitting is commonly used in plumbing and sewage applications due to its versatility and ease of installation in tight spaces.

In contrast, standard elbows typically feature the same type of thread on both ends, reducing the ability to adapt to varying thread types. Reducing elbows change the size of the pipe or fitting, while adapter elbows are specifically designed to connect pipes of different materials or sizes without going into the specifics of thread types. Hence, the street elbow is the correct terminology for this specific fitting configuration.