Which type of plumbing fitting is used to connect two pipes of different diameters?

The appropriate plumbing fitting used to connect two pipes of different diameters is a reducing coupling. This type of coupling accommodates the size variation between the two pipes by tapering between the two different diameters, ensuring a secure and leak-free connection. This fitting is particularly important in plumbing systems where different sized pipes must be linked, such as transitioning from a larger main line to a smaller pipe leading to a fixture, thus allowing for efficient fluid flow and maintaining pressure levels.

Straight couplings are designed to connect two pipes of the same diameter without any size adjustment, making them unsuitable for this situation. T-fittings are utilized to create a branch off the main line, typically at the same diameter or where a diversion is needed but do not serve to connect pipes of differing sizes. Elbow fittings allow for directional changes in piping but also do not accommodate size differences in the pipes being connected. Therefore, the reducing coupling is specifically engineered for linking pipes of varied diameters, making it the correct choice.