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Besides the material used to manufacture a fitting there are several other factors that need to be known in order to identify the right fitting.  Thread size & thread pitch, swivel or fix, type of seal and are the threads tapered or parallel are a few of the things needed to be known.

If the threads are tapered, you can look at the thread profile and see the taper.  These hose connector types of fittings seal on threads as the fitting is tightened.  A thread sealant is needed for a leak free connection.  National Pipe Thread (NPT) and British Standard Pipe Taper (BSPT) are the most common in this style.

Besides sealing on the threads like the tapered thread fittings there are several different style sealing methods available.  These steel hose fitting connector seals are different than the taper thread fittings.  The threads are usually parallel and seal on flares on the nose of the fitting, o-rings at the base of the threads, o-rings on the nose of the fitting, o-ring held using a flange and a sleeve biting into a tube.

These links will help you to determine the steel pipe fitting style you are looking to buy.