Former New York City mayor Rudolph W. Giuliani speaks with members of the media in the lobby of the Trump Tower in New York in November 2016. (Behar Anthony/European Pressphoto Agency)

Former New York City mayor Rudolph W. Giuliani told CNN’s Dana Bash during the weekend that he already knew most of the evidence special counsel Robert S. Mueller III had in his possession.

“I have a pretty good idea because I have seen all the documents that they have,” Giuliani said. “We have debriefed all their witnesses. And we have pressed them numerous times.”

“You have debriefed all of their witnesses?” Bash asked.

“I think so, I mean, the ones that were — the ones that were involved in the joint defense agreement, which constitutes all the critical ones,” Giuliani replied.

That term, “joint defense agreement,” is one with which many people may not be familiar. Given the evolution of Mueller’s investigation into the 2016 campaign and the effort by President Trump and his team to push back on it, though, it’s worth understanding.

“A joint defense agreement is just an agreement that various defendants are going to share what would otherwise be confidential information,” said Maria Glover, professor of law at Georgetown University, when we spoke by phone Monday. “Normally anything that you say to your attorney in the course of legal representation is protected from disclosure under the attorney-client privilege.” A joint defense agreement essentially extends that concept outward to other parties and attorneys.

Not to just anyone, mind you. You can’t simply declare that your friend Bill is part of a joint defense agreement with you and therefore can’t be made to testify about a conversation the two of you had.

“The joint defense agreement is created by the existence of a common legal interest that would enable defendants, if they agree among one another to keep information confidential that is shared, that that common legal interest applies to give them that attorney-client protection,” Glover said. “So if you say something to your co-defendant’s attorney in the context of a joint defense agreement, your communication is protected with regard to that other attorney as if he were your own attorney.”

This doesn’t simply cover testimony. Like attorney-client privilege, it covers a broad variety of communication over the course of a legal fight, including well before any charges are filed.

Who is included in Trump’s joint defense agreement? It’s hard to say. Reporter Marcy Wheeler tried to suss out the possible participants in Trump’s. Take former White House communications director Hope Hicks, for example: Some of her legal bills were paid by the Republican Party, hinting she’s possibly party to it. It’s hard to say with certainty, and the White House hasn’t articulated specifically who’s included. Given Giuliani’s comments and the number of people who share common attorneys, though, it probably does encompass most of the main players at stake: Donald Trump Jr., Jared Kushner, Stephen K. Bannon, etc.

“I mean, those are attorney-client privilege, and it involves a lot of other people,” Giuliani said to Bash. “So, yes, there is a joint defense agreement, but I don’t think I can identify all the people who are in it, people who are out of it.”

He was responding to a question from Bash about one specific member: Trump’s former personal attorney Michael Cohen. It was reported last week that Cohen would be leaving the agreement.

“If somebody gets indicted, they’re basically out of it,” Giuliani said when asked whether Cohen was already out. “So, I think I can’t go much further than that.”

“It gets a little bit dicey because the agreement doesn’t really hold when the common legal interest breaks down,” Glover said. “That’s always the risk: You’re in common legal interest for now.” She put it succinctly: Often, “defendants make strange bedfellows.”

Our Aaron Blake noted that we’ve seen this script before: Shortly before former national security adviser Michael Flynn agreed to cooperate with Mueller’s team, he left Trump’s joint defense agreement. In that case, absent a formal agreement aimed at protecting confidentiality, Flynn could have shared any information he wanted with Mueller’s team, including information he learned by talking to other attorneys included in the broad pool encompassed by the agreement.

In Cohen’s case, it’s trickier. Because Cohen was Trump’s attorney, he is still bound by attorney-client privilege for conversations he had with Trump. There’s an important exception, though: Communications between an attorney and a client that were meant to commit or cover up a crime or an act of fraud aren’t considered privileged.

It’s important to note that the joint defense agreement applies only to those whose inclusion can be defended. In other words, Glover explained, including Trump Jr. in the agreement and claiming that his communications with Trump’s attorneys are privileged could be challenged by Mueller’s team. Each side would make its case for why Trump Jr. should or should not be covered under that agreement, and a judge would eventually make the determination of whether privilege applied. Trump Jr. reportedly invoked a privilege claim when testifying before the House Intelligence Committee last year; a similar claim meant to shield him from Mueller could face a legal challenge.

One reason Giuliani may not be willing to identify everyone covered under the agreement is that its members are nebulously identified. The agreement doesn’t need to be formal, Glover said, because it’s an extension of the attorney-client privilege doctrine. That said, though, attorneys will often create a written document memorializing participants and expectations.

Why?

“Lawyers are careful,” Glover said.