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There are both black steel and galvanized steel couplers on the market. When it comes to steel, stainless galvanized steel couplers are what we recommend, as this kind of steel plate will resist rust better and longer than black steel couplers. Couplers come in all sizes to fit one galvanized pipe to another.
While searching for ways to connect two threaded pipes, especially if you're connecting a gas line, you may have come across the term "teeing" which refers to using a three-sided coupler, or a tee.
We always recommend consulting a professional installer when making connections with gas fuel lines.