One of China’s biggest copper fabricators is set to reap a windfall from President Donald Trump’s efforts to boost US production of the metal.

The president stopped short of putting a 50 percent tariff on refined copper, as he had threatened earlier this year. The market reaction was swift anyway and prices tumbled.

The 50% tariff on imported copper that starts Friday excludes a critical input that companies say they can’t get enough of in the U.S.: cathodes. Here's what you need to know. What are cathodes? Cathodes are a key intermediate product in copper manufacturing.