The couplings are main used to protect long lengths of pipe that has threads on the end from damage.  They are smooth on the outside devoid of markings without visible seams to assist is easy stacking of pipe.  They are usually straight threaded to allow the couplings to spin down onto the pipe rather than bind up if they were tapered.  On pipe diameters 2-1/2" and up, they are tapered.  They are available in black or galvanized finishes.  They can be installed in a low pressure system.  Merchant couplings have threads on both sides that can connect two lengths of pipe.