A coupling connects two pipes. If their dimensions differ, the fitting is referred to as a lowering coupling, reducer, or an adapter. There are 2 kinds of couplings: "frequent" and "slip". A daily coupling has a small ridge or stop internally, to circumvent over-insertion of the pipe, and so underneath-insertion http://valvesandfittings07262.blogofoto.com/16874861/not-known-facts-about-valves-and-fittings