Plugs, stoppers and inserts fit into holes or cavities and are designed for the ends of shafts, tubes, pipes, fittings, connectors, screws, bars, or other shapes. Plugs are available in many different shapes and materials.  Plugs come in a variety of different cross sections.  These include round, button plug, square, rectangular, and screw cap or plug.  Common features include threaded, tapered, and expanded tube plug.  Threaded plugs, stoppers, and inserts have a female thread.  The thread can be straight or tapered (NPT).  Tapered sides provide a snug fit.  Some can be used as either a cap or a plug.  Tapered caps and plugs are versatile and can be used for straight tubes and bars, male and female NPT threads, and male straight threads.  Expanding tube plugs are used in moderate pressure applications, such as condenser and heat exchanger tubes.