(Jabin Botsford/The Washington Post)

Donald Trump’s presidency has prompted early and widespread speculation of its end through resignation, removal or a finding of presidential inability. Whatever the plausibility or merits of such scenarios, each would involve the 25th Amendment to the Constitution, which makes it clear that the vice president will take over in any of those events and, following a resignation or removal, would also nominate his successor.

Here are five things you should know about this often overlooked amendment.

Illness and assassination prompted the amendment’s consideration in the 1960s

The 25th Amendment was conceived and created during the Cold War, prompted by President Dwight D. Eisenhower’s three serious illnesses, President John F. Kennedy’s assassination in 1963, and the vice presidency’s midcentury evolution. After a serious heart attack, a briefly crippling stroke and a major surgery, Eisenhower worried that the nation might face a crisis while he was unable to respond. He entered into a letter agreement with his vice president, Richard Nixon, that would temporarily transfer power to Nixon, if necessary.

But that was just a short-term fix. For a more enduring solution, Eisenhower proposed amending the Constitution to clarify that a vice president could step in temporarily for an incapacitated president, and providing procedures for determining presidential inability.

After Kennedy’s assassination, Sen. Birch Bayh (D-Ind.), as the new chairman of the Senate subcommittee on constitutional amendments, embraced the mission of solving the problems of presidential succession and inability. Bayh combined a fortified version of the Eisenhower administration’s proposal regarding presidential inability with a new way to fill vice-presidential vacancies without having to wait for the next scheduled presidential election and inauguration, as had been the case.

Congress proposed the amendment in 1965, and it was ratified on Feb. 10, 1967.

Section 1: The amendment updated the Constitution to reflect historical practice

The original presidential succession clause had treated presidential death, resignation, removal and inability identically, passing all presidential powers and duties to the vice president, if that office was occupied. The framers probably didn’t intend to pass along the presidency itself — but when President William Henry Harrison died in April 1841, Vice President John Tyler insisted that he became president, not simply acting president. Tyler’s interpretation, though probably wrong, was ultimately accepted and followed when seven other presidents died in office (Zachary Taylor, Abraham Lincoln, James A. Garfield, William McKinley, Warren G. Harding, Franklin D. Roosevelt and, of course, JFK).

But the Tyler precedent complicated the question of how to handle presidential inability by implying that transferring presidential powers and duties from a disabled president — even briefly — would permanently displace him, since that’s what the Tyler precedent called for in the other three situations. The 25th Amendment clarified that in its first section, by explicitly stating that if a president dies, resigns or is removed from office, the vice president becomes the president. As will be seen below, it treated presidential inability differently.

Through that first section, Gerald R. Ford became president when Nixon resigned.

Section 2: The president and Congress share the power to replace a vice president

The second section allows the president to fill a vice-presidential vacancy by nominating a new vice president who takes office after being confirmed by both houses of Congress. The founders’ opinion of the position was suggested by the fact that they didn’t see a need to address a vacancy in the office; its duties were simply reassigned. But by the mid-1960s, the vice presidency had become more significant than the founders had envisioned and was widely seen as the best way to handle presidential succession or inability. Allowing the president to nominate the VP followed the practice of allowing presidential nominees to choose their running mates. Having that nominee confirmed by Congress simulated an election of sorts.

In other words, Section 2 made it clear that the president was entitled to a personally and politically compatible vice president — albeit subject to deliberation by politically accountable figures. Under this provision, Ford and then Nelson A. Rockefeller became vice presidents in the mid-1970s, after Vice President Spiro T. Agnew resigned, and then after Ford stepped up to replace Nixon.

Before 1967, the vice presidency was only filled by the next election, so it was left vacant for 37 years, or about a fifth of the time since the Constitution was adopted. Since the 25th Amendment’s ratification, it’s been vacant for six months, or less than 1 percent of the time.

Section 3: Disabled presidents can temporarily cede power and duties to the vice president

The third section allows the president to temporarily transfer presidential powers and duties to the vice president, and to reclaim those powers when he is again able to discharge them. This procedure has been used three times — by Ronald Reagan in 1985 and by George W. Bush in 2002 and 2007 — transferring power briefly to their vice presidents as they underwent surgery under general anesthesia.

At least four other times, Jimmy Carter, Reagan, George H.W. Bush and Bill Clinton made plans to transfer power under Section 3 if they had to go under general anesthesia. But either the planned medical procedure or the strong sedation wasn’t needed.

Section 4: The amendment empowers the vice president and Cabinet to declare a president incapacitated

The section that some have recently discussed addresses the problem of a president who is unable or unwilling to acknowledge his or her inability “to discharge the powers and duties” of the presidency.

Under current law, the vice president and a majority of “the principal officers of the executive departments” — which the legislative history makes clear are essentially the Cabinet officers listed in the line of presidential succession — may declare the president incapacitated by a written notice to the speaker of the House and the president pro tempore of the Senate. At that point, the vice president automatically takes over presidential powers and duties as acting president.

This section seems most likely to be used when there’s an unexpectedly unconscious president — although it clearly applies if a president is incapacitated from some other mental or physical inability.

The president can reclaim those powers and duties by declaring his or her ability, again in writing, to those same legislative leaders. But that claim is open to challenge: Within four days, the vice president and majority of the Cabinet may again reassert that the president is “unable to discharge the powers and duties” of the presidency. If that happens, Congress essentially has 21 days to decide which one is right.

The bar for Congress declaring a president disabled is high. The president resumes power unless two-thirds in each chamber conclude he or she “is unable” to handle the office. The amendment and its history make clear that the vice president continues to act as president during both the four-day waiting period and the 21-day deliberation period — unless either the group of executive officials or at least one house of Congress decide sooner that the president can “resume the powers and duties” of the office.

The amendment allows Congress to replace the Cabinet as part of the decision-making process with “such other body as Congress may by law provide.” Congress hasn’t created an alternative body. But if it does, that group would supplant the Cabinet, although the vice president would stay involved.

Section 4 has not yet been used.

Joel K. Goldstein is the Vincent C. Immel Professor of Law at Saint Louis University School of Law and author of “The White House Vice Presidency: The Path to Significance, Mondale to Biden.” (University Press of Kansas, 2016).