What is the name of the fitting that is needed to join two ends of threaded pipe that cannot be rotated?

The correct answer, which refers to the type of fitting used to join two ends of threaded pipe that cannot be rotated, is a union. A union is specifically designed to connect two pieces of pipe while allowing for disconnection and reassembly without rotating the entire assembly. This makes it particularly useful in situations where space is limited, or when the pipes are in a fixed position and cannot be rotated for installation.

The union comprises three parts: two end pieces that are threaded onto the pipe ends and a central nut that holds them together. This design allows for easy removal and reinstallation of pipes without the need to unscrew or rotate the entire fitting, making maintenance tasks more accessible.

In contrast, a coupling is a fitting that joins two pipe ends in a straightforward manner without the ability to disconnect easily, as it typically requires that the pipes are unscrewed to separate them. A nipple is a short piece of pipe with threaded ends used to connect two other fittings or pipes together. An elbow is a fitting that changes the direction of the piping system, allowing for bends in the line, but it does not assist in connecting two ends of pipe in a way that accommodates fixed positions without rotation.