The Mueller report is full of lawyers doing lawyerly things (and some very unlawyerly things). But no anecdote stands out as a more basic illustration of the role of an attorney as Don McGahn’s exchange with President Donald Trump over—note-taking.

During his conversations with Trump, White House lawyer Don McGahn, reportedly took notes. At one point, Trump asked him, “What about these notes? Why do you take notes? Lawyers don’t take notes. I never had a lawyer who took notes.”

McGahn, who is now back at his old law firm Jones Day as a partner, responded that he took notes because he is a “real lawyer.” The president pushed back, saying that “I’ve had a lot of great lawyers, like Roy Cohn. He did not take notes.”

But McGahn is right. Lawyers have many reasons to routinely take notes as part of their job, whether to help aid their memories, log information for colleagues, or protect themselves against charges of wrongdoing.

“I cannot imagine being a lawyer and not taking notes,” said Linda A. Klein, a shareholder at Baker Donelson and past president of the American Bar Association.

In fact, her clients expect her to do so, she said. “You don’t have to be a sophisticated corporate client to understand that if you’re telling your lawyer something, you don’t want them to forget it.”

“The primary purpose of taking notes is making sure everything you need to know to protect your client is there,” she added. “What if your lawyer gets run over by a truck? Somebody has got to be able to pick up the file and run with it.”

Stephen Gillers, professor of ethics at NYU Law, said lawyers also take notes in case others in a conversation recall things differently.

“Contemporaneous notes are highly credible,” he said.

“And they take notes to protect themselves in the event they are later sued or charged with wrongdoing,” Gillers added.

Lawyers know that “the law provides protections for confidential notes,” whether through attorney-client privilege or work product immunity, stressed White & Case partner Carolyn Lamm, another former ABA president.

“There may be instances when lawyers do not, as agreed with clients, take notes, to avoid creating even potentially discoverable evidence,” Lamm said.

For example, in litigation resulting from the Bridgegate scandal, lawyers for former New Jersey Gov. Chris Christie said in 2015 that they didn’t take notes because they were worried about journalists and legislators getting their hands on them.

But those instances are rare. Most of the time, lawyers do, in fact, write things down, which Trump still finds objectionable.

“Watch out for people that take so-called ‘notes,’ when the notes never existed until needed,” he said in a Tweet on Friday.