Reducers are used to connect plumbing segments of different diameter. At right, is a typical galvanized steel bell reducer, connecting a 1 and 1/2-inch galvanized pipe nipple mounted in a commercial water heater (a typical size) with 1-inch galvanized plumbing.
They can also be made of brass, and there are solderable copper pieces made for changing diameters on a copper plumbing line.