Couplings are Fittings used to connect two pipes together. Couplings come in different classes depending on the pressure within the piping system. The ends of the coupling come in either threaded or socket-weld forms, to match the input pipe.
Half-Couplings are the same as Couplings, however the length varies: for threaded pieces, it is exactly one half that of a full size coupling; for socket pieces it is slightly smaller, as the channel is elongated. As a result, only one side is threaded or socket-weld ready: the other is smooth.