The refrigeration industry uses a bewildering range of connectors including various threads, flares, "O" rings, compression and unusual combinations of all of the above.  Chances are good that anyone working on a system, particularly an older system, is going to be faced with the challenge of try to find an elegant way to attach one odd thread or flare to some other equally obscure (but invariably different)  thread, flare or "O" ring.  Those of you who have been through this before and don't want to do it again will love the useful  collection of adapters below.