 Compression fittings (sometimes called "lock-bush fittings") consist of a tapered, concave conical seat; a hollow, barrel-shaped compression ring (sometimes called a ferrule); and a compression nut which is threaded onto the body of the fitting and tightened to make a leakproof connection.They are typically made of brass or plastic, but stainless steel or other materials may be used.Although compression connections are less durable than soldered (aka sweated) connections, they are easy to install with simple tools.