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 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. However, they take longer to install than soldered joints and sometimes require re-tightening to stop slow leaks which may develop over time. Because of this possible leakage, they are generally restricted to accessible locations (such as under a kitchen or bathroom sink) and are prohibited in concealed locations such as the interiors of walls.