Beijing (CNN) President-elect Donald Trump had a 10-minute phone conversation with Taiwan's president Tsai Ing-wen on Friday.

It's not unusual for the incoming leader of one government to have a chat with the sitting leader of another, if only to exchange pleasantries, but Trump's call with Tsai is causing a huge diplomatic storm.

A lot of speculation is swirling around about what it means for the future of U.S. relations with China and Beijing's relations with Taipei. Here's why:

1. Who's the boss?

China and Taiwan both agree there is only one nation of China. But the jury is still out on who the legitimate ruler actually is. It has been this way since 1949 after the communists won the civil war and forced the nationalists, who previously ruled, to flee the mainland to the island of Taiwan. The communists set up a new government with the capital in Beijing, officially proclaiming the birth of the People's Republic of China (PRC). The nationalists set up their own government in Taiwan still officially known as the Republic of China (ROC). In practice, the PRC governs most of the Chinese territory and the ROC governs Taiwan and a few smaller islands near it. Most countries recognize the PRC as the legal ruler of all of China.

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