It's been called the world's most exclusive fraternity: the men who have served in the Oval Office. Out of respect, members largely avoid criticizing one another, regardless of political party.
But now there's a new pledge, and Donald Trump is writing his own rules.
The soon-to-be 45th president took to Twitter on Tuesday to lay into Bill Clinton, saying the 42nd president had misrepresented a recent phone call between them in remarks reported by a newspaper that were sharply critical of Trump. He also tweeted that Clinton didn't know much about motivating voters in key swing states and that Hillary Clinton's campaign had "focused on the wrong states."
The broadsides were just the latest example of the Republican president-elect using Twitter in unprecedented ways, including provoking China and musing about possible punishments for flag-burners.
This time, he and Clinton broke with a long tradition of U.S. presidents treating one another with kid gloves, at least outside political campaigns, presidential historians said.
"That's one of those many, many unwritten rules otherwise known as norms that Donald Trump has abandoned," said Thomas Mann, a scholar at the Brookings Institution.
Trump was apparently provoked by an editorial in the Record Review, a small-town weekly newspaper that serves readers near the Chappaqua, New York, home of Bill and Hillary Clinton.
The paper's editor, Ed Baum, wrote that he and his wife had gone into a local bookstore on Dec. 10 only to find the former president also doing what appeared to be holiday shopping.
For more than 35 minutes, Bill Clinton held court, answering question after question from patrons.
"It was a setting he was comfortable in, and he felt at ease taking questions from the group, and he responded the way he wanted to," Baum said in an interview.
The article paraphrased Clinton as saying that he had "received a phone call from the president-elect the day after the election" and that Trump was cordial "like it was 15 years ago," when the Clinton and Trump families might be seen socializing.
Then, the paper said, Clinton was asked by an unnamed man if Trump is smart.
"He doesn't know much," Clinton replied, according to the report.
"One thing he does know is how to get angry, white men to vote for him."
No one in Clinton's orbit has disputed the encounter.
A Clinton spokesman declined to comment.
"He appeared to still be wrestling with the election results and trying to come to terms with it," said Baum, who said he stood by and observed the encounter, taking notes.
Baum said that during the course of the conversation, he identified himself to Clinton as a newspaper editor. The session was not recorded, he said.
Trump fired back to his 17.6 million Twitter followers: "Bill Clinton stated that I called him after the election. Wrong." The former president did offer "a very nice congratulations," Trump allowed.
But then he continued the criticism. Clinton, Trump wrote, " 'doesn't know much' . . . especially how to get people, even with an unlimited budget, out to vote in the vital swing states (and more)."
"They focused on wrong states," Trump added.
Trump's electoral college victory was propelled by unexpected wins in the traditionally Democratic-leaning states of Pennsylvania, Michigan and Wisconsin; the latter two were not frequent campaign stops for Hillary Clinton.
Later Tuesday, Bill Clinton agreed that he had in fact called Trump - not the other way around.
"Here's one thing @realDonaldTrump and I can agree on - I called him after the election," Clinton said in a tweet of his own.
The level of warmth among presidents has varied, but at the very least, the relationships have been characterized by mutual respect and understanding shared among the few - all of them men, so far - who have occupied the office.
"I think it comes from the empathy of having felt the pain as well as the glory, and realizing that it's difficult work and it's something you share with very few people on earth," said Bruce Buchanan, a presidential historian at the University of Texas at Austin.
"Former presidents are very sympathetic and supportive of each other."
But like many things during and after this campaign, the Trump-Clinton relationship is complicated in a way not fully captured by considering them only as two men in the exclusive club of presidents.
They ran in similar New York social circles after Hillary Clinton became the state's U.S. senator. Over the years, Trump donated $100,000 to the Clinton Foundation and to Clinton. In turn, the Clintons attended Trump's wedding to his third wife, Melania, at his Mar-a-Lago resort in Florida in 2005. One photo capturing the foursome caught up in a moment of good humor remains an artifact of that moment in their history.
A few years later, in a 2008 interview, Trump heaped praise on Hillary Clinton after she ran for the presidency and lost the Democratic nomination to Barack Obama.
"I think she's going to go down at a minimum as a great senator," Trump said at the time.
"I think she is a great wife to a president, and I think Bill Clinton is a great president."
As they faced off in the 2016 race, both sides disavowed any past chumminess. Hillary Clinton repeatedly characterized her association with Trump as casual. And Trump declared it a product of his need to sweeten up politicians on both sides of the political aisle for the good of his businesses.
Today, the election wounds are still very fresh. And it isn't clear who is most responsible for extending the olive branch: the victor or the spouse of the political loser, Buchanan said.
"If Bill Clinton didn't have as his spouse the person who ran against Trump, I don't think you would have heard him talk like that," Buchanan said.
"The comments Trump made about Hillary - 'crooked Hillary' and that sort of thing - really irritated Clinton."
Just Monday, Bill Clinton served as an elector in New York for his wife, tweeting afterward that "I've never been more proud to cast a vote than my vote today for @HillaryClinton."
Buchanan said Trump hasn't yet learned to adjust his style to his new job.
"The incumbent has to put his deployment of the presidency on a somewhat more elevated plane than that," he added.
"I don't think he's going to enjoy the kind of reactions he's going to get if he acts like a defiant teenager caught up in a catfight when he's having a discussion with someone he's dealing with."
It's not unprecedented for a former president to question the views of a sitting one. Richard Nixon, for example, was openly critical of President Ronald Reagan's strategy toward the Soviet Union, said Timothy Naftali, a presidential historian at New York University.
But, he said, "Nixon was attacking Reagan's policy. He wasn't attacking his brain. It's not this kind of personal nonsense."
Asked whether Clinton started the recent spat, Mann said he shouldn't shoulder the blame.
"He didn't go out and make a public statement," Mann said. "It was a conversation in a bookstore. Usually, you ignore that kind of thing."
Naftali offered a similar take but said Clinton should know that, in the age of social media, comments he thinks are private can easily become public.
"I think he has a right to grouse," Naftali said, "but he should grouse more carefully."