Couplings are fittings used to join two or more lengths of pipe in a straight run. If the material and size of the pipes being joined are not the same, the fitting may be called a 'reducing coupling' or reducer, or an adapter. The term 'expander' is not used for a coupling that increases pipe size; instead 'reducer' is used.
Couplings for copper pipe are available either with or without a stop. The stop is in the center of the coupling and is useful for ensuring that the coupling is centered between the two pipes being connected. The disadvantage of a stopped coupling is that in some repair situations it may be necessary to slide the coupling completely on to one pipe and then slide it back when the pipes being joined are in position. The type of coupling that doesn't have a stop is referred to as a repair coupling. Repair couplings are available in longer than standard lengths to facilitate the repair of a short length of pipe.
There is a wide range of coupling types, including threadless couplings, flexible couplings, compression couplings, split couplings and quick-connect couplings. Couplings are used with many metal and plastic pipe and tubing materials.