(RNN) – We all know the vice president steps in if the president can't serve.
Most of us know the speaker of the House of Representatives takes over if the vice president can't.
But what happens if the speaker can't fill the office?
The presidency then goes to an official who's already a president of sorts - president pro tempore of the U.S. Senate.
From there, the line of succession goes through eligible heads of federal executive departments who form the president's cabinet.
That's according to the Presidential Succession Act, signed by President Harry Truman in 1947.
Here are the 18 officials in line for succession, in order:
- Vice President Mike Pence
- Speaker of the House Paul Ryan
- Senate President Pro Tempore Orrin Hatch
- Secretary of State Mike Pompeo
- Secretary of the Treasury Steven Mnuchin
- Secretary of Defense James Mattis
- Attorney General Jeff Sessions
- Secretary of Interior Ryan Zinke
- Secretary of Agriculture Sonny Perdue
- Secretary of Commerce Wilbur Ross
- Secretary of Labor Alexander Acosta
- Secretary of Health and Human Services Alex Azar
- Secretary of Housing and Urban Development Ben Carson
- Secretary of Transportation Elaine Chao
- Secretary of Energy Rick Perry
- Secretary of Education Betsy DeVos
- Secretary of Veterans Affairs Robert Wilkie
- Secretary of Homeland Security Kirstjen Nielsen
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