Former U.S. Presidents Richard Nixon, left, and Gerald Ford, meet at the White House during the funeral for former Vice President Hubert Humphrey in 1978. (White House photo via Wikimedia Commons)

Today is the 49th anniversary of the ratification of the 25th amendment to the Constitution of the United States.

Here are some things you may not have known about presidential succession.

Before the 25th amendment was ratified, the U.S. Constitution was unclear regarding presidential succession in cases of death, resignation or inability to serve. The first president to die in office was William Henry Harrison in 1841. There was controversy over whether his vice president, John Tyler, automatically became acting president or succeeded to his own presidency. Tyler took the oath of office and assumed full presidential duties. Both houses of Congress resolved that Tyler was president in his own right, establishing what came to be known as the “Tyler Precedent.”

The Constitution also didn’t provide a way to fill a vice presidential vacancy until the next presidential election. When Franklin Roosevelt died months after being sworn in for his fourth term as president, his successor, Harry Truman, was not able to choose a replacement for himself as vice president for nearly four years.

There was also the problem of what to do when a president becomes incapacitated. In 1919, President Woodrow Wilson suffered a stroke, which left him partially paralyzed. His wife decided which matters would be brought to the president’s attention and which could be handled by cabinet members. As there was no mechanism for removing an incapacitated sitting president from power, Wilson was allowed to serve out the remaining year of his term.

The 25th amendment states specifically that if the president dies or resigns, the vice president becomes president in his own right.

It also clears up the matter of vice presidential succession; it states if there is a vice presidential vacancy, the president will nominate a successor who needs to be confirmed by a majority vote of both houses of Congress.

It also allowed for the president to transfer his powers and duties to the vice president if he is unable to discharge them; such as in cases of planned medical procedures.

Finally, the amendment allows the vice president and a majority of the members of the cabinet to transfer the powers of the acting presidency to the vice president in the event of the president’s incapacitation. The president is then allowed to retake his power by notifying Congress. However, the vice president and cabinet may override that declaration, which would then require a two-thirds vote of both houses of Congress to keep the president from regaining his powers.

Since its ratification in 1967, the first three sections of the 25th Amendment have been invoked. The first three times, both of which invoked the first two sections of the amendment, involved the administration of Richard Nixon.

In 1973, Gerald Ford was selected to replace Spiro Agnew as vice president. In 1974, Ford succeeded Nixon to the presidency, and Nelson Rockefeller succeeded Ford to the vice presidency.

The third section has been invoked three times, all involving presidents undergoing colonoscopies. In 1985, Ronald Reagan transferred powers to Vice President George H.W. Bush while undergoing surgery to have a pre-cancerous lesion removed following a procedure. The other two times involved George W.