The first iteration of the Presidential Succession Act in 1792 stated the Senate president pro tempore would be next in line after the vice president. On July 18, 1947, President Harry Truman signed the Presidential Succession Act of 1947. This act changed the line of succession in two ways; first it reinstated the Senate president pro tempore and speaker of the House to the succession plan. Secondly, it also placed the speaker of the House right after the vice president instead of the Senate president pro tempore. The last change to the act came in 2006, when the USA Patriot Improvement and Reauthorization Act of 2005 added the secretary of Homeland Security to the list.

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In Trump's administration, here's the full line of succession.

The only caveat here is that Secretary of Transportation Elaine Chao isn't eligible since she was born in Taipei, Taiwan, and is therefore not a US-born citizen. While it's still uncertain whether or not we'll need to refer to this list in the future, at least we know who we can expect.