WASHINGTON — (AP) — Ronald Reagan probably didn't realize he was starting a tradition when he wrote a note congratulating his successor and left it in the Oval Office desk drawer after two terms as president.

He did that for George H.W. Bush, his successor and vice president of eight years. Bush did the same for Bill Clinton, who left a note for Bush's son, George W. The younger Bush left behind written words for Barack Obama, who later put pen to paper for Donald Trump.

Trump curiously continued this rite of presidential passage by writing a letter to Joe Biden, even as he opted out of other traditions, like attending Biden's inauguration.

History and politics now have intertwined to put President Biden in the unique position of writing a letter — if he so chooses — to Trump, his successor and the predecessor who left a note for him.

“This will mark the first time that a president who has received a letter from an outgoing president may well be writing a letter to the same person who’s the incoming president," said Mark Updegrove, president and CEO of the LBJ Foundation.

When Trump takes office on Monday, he'll be the first president to serve nonconsecutive terms since Grover Cleveland in the late 1800s, when the letter-writing tradition didn't exist.

"So this is a highly unusual situation, as so many things are in modern day Washington with Donald John Trump,” Updegrove said in an interview.

Reagan was inspired to write to George H.W. Bush, who had become a friend during their eight-year partnership, Updegrove said.

He chose a sheet of whimsical stationery illustrated by the cartoonist Sandra Boynton with an elephant — also the Republican Party mascot — surrounded by turkeys and the phrase, “Don’t let the turkeys get you down.”

“Dear George,” the 40th president wrote in January 1989, opening the two-paragraph note. “You’ll have moments when you want to use this particular stationery. Well, go to it.”

Reagan writes that he treasures the memories they share and “wish you all the very best.” He closed with, “I’ll miss our Thursday lunches,” and signed it, “Ron.”

The tradition was elevated, Updegrove said, when the elder Bush turned over the presidency after one term, denied a second by Bill Clinton in the 1992 election.

Bush used his note to wish Clinton “great happiness here” in the White House. He warned of tough times that will be made more difficult by criticism Clinton will think unfair, and he advised the man who defeated him to “just don't let the critics discourage you or push you off course.”

“Your success now is our country’s success. I am rooting hard for you,” Bush wrote.

“It’s just a very bipartisan, genuine reflection of, I think, really, George H.W. Bush’s character,” Matthew Costello, chief education officer at the White House Historical Association, said during an online program about inaugural traditions.

At the end of his two terms, Clinton told the younger Bush that the incoming president was embarking on the “greatest adventure, with the greatest honor, that can come to an American citizen” and wished him “success and much happiness.”

“The burdens you now shoulder are great but often exaggerated. The sheer joy of doing what you believe is right is inexpressible,” Clinton wrote.

Eight years later, Bush congratulated Obama on opening a “fantastic chapter in your life” but warned of the trying moments ahead with critics who “rage” and “friends” who will disappoint.

“But, you will have an Almighty God to comfort you, a family who loves you, and a country that is pulling for you, including me,” George W. Bush wrote.

Obama, who had campaigned vigorously against Trump in 2016, congratulated him on a “remarkable run” and offered some reflections from his eight years on the job.

He told Trump they were both blessed with good fortune, that American leadership “really is indispensable” in the world, that they are the “guardians” of democratic institutions and traditions, and that family and friends will see him through the “inevitable rough patches.”

“Millions have placed their hopes in you, and all of us, regardless of party, should hope for expanded prosperity and security during your tenure,” Obama wrote.

What did it say? Hardly anyone knows because the letter has not been shared with the public.

Biden showed the note to some members of his staff after his inauguration in 2021 but didn't let anyone read it. Others described Trump’s note as long and handwritten, and Biden is said to have been surprised by how gracious he thought the letter was given the animosity between the political rivals.

Asked about the note later that day, Biden said it was a “very generous letter," but he deemed it private and said he wouldn't discuss it until he had a chance to speak with Trump.

Trump said he thought it was up to Biden to share the letter.

“It was a nice note," he said during a September 2023 interview with NBC's “Meet the Press,” adding, "I took a lot of time in thinking about it.”

He's expected to, as someone who respects political traditions and institutions. But White House press secretary Karine Jean-Pierre left open the possibility that he might not.

“That is going to be up to the president,” Jean-Pierre told reporters this week. “It’s a good question. I’m curious, too.”

Letter-writing isn't required but is “based on precedent and whether or not people want to keep a tradition going," Costello said.

He said he was hopeful that Biden would participate.

Some earlier outgoing presidents wrote letters to their successors, though not on Inauguration Day and not always with congratulations in mind, Costello said.

Earlier notes often were penned with invitations to visit or dine at the White House or to relay information, which is what President John Adams did in a February 1801 letter to inform incoming President Thomas Jefferson about transportation.

“In order to save you the trouble and expence of purchasing horses & carriages, which will not be necessary, I have to inform you that I shall leave in the stables of the United States seven horses and two carriages with harness the property of the United States,” Adams wrote. “These may not be suitable for you, but they will certainly save you a considerable expence as they belong to the studd of the Presidents Household," Adams wrote.

“I have the Honor to be with great respect, Sir your most obed. & hum servt.”

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AP White House Correspondent Zeke Miller contributed to this report.

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