Black pipe fittings (black malleable iron fittings) are used to make connections to black iron pipe.  Malleable fittings are those fittings having the property of malleability. This is a physical property of metals and metalloids, or any kind of matter. We call a metal malleable when it can easily be deformed, especially by hammering or rolling, without cracking the metal.  Black iron piping is one of the best options for gas lines that exists however it is also used in oil and air applications.
VMS stocks black malleable fittings in schedule 40 and 80 and in sizes ranging from 1/8” through 2” and in all available shapes including unions, changes of direction (bends, elbows), branches (tees), bushings, extensions, plugs, nuts, reducers, couplings etc., allowing for unlimited possibilities of connection.