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When Donald Trump and Hillary Clinton face off for their first debate on Monday, a strict set of rules are designed to govern the event. Besides the candidates themselves, however, virtually no one actually knows what the rules say.

It's a curious tradition in general election debates: The candidates’ top strategists negotiate written rules and keep them a closely guarded secret.

Those rules range from the trivial, such as how the candidates enter the room, to potentially pivotal restrictions on whether the moderator should correct false statements by the candidates.

Related: Debate Details Revealed: Clinton Gets First Question

And while the Commission on Presidential Debates on Sunday morning announced some details about the basic timing of the debates, many potential other rules on big controversies are still unknown or undisclosed.

This year, in a season of intense scrutiny on media moderators and complaints about a "rigged system," the debate structure could draw more attention than ever.

Related: Meet the Man Role-Playing Trump for Clinton's Debate Prep

Donald Trump laid down his line in a Fox News interview, arguing journalists leading the debates should be moderators — not fact-checkers.

"You're debating somebody, and if she makes a mistake or if I make a mistake, we'll take each other on," he said.

"I certainly don't think you want Candy Crowley again," Trump added.

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During a crucial moment in the 2012 debates, CNN's Crowley famously rebutted an assertion by Mitt Romney, enraging supporters who argued the point — about how Barack Obama characterized terrorism after the Benghazi attack — was muddled at best.

The ensuing firestorm led to a rare leak of that year’s rules.

The 21-page memorandum, negotiated by Obama lawyer Bob Bauer and Romney lawyer Ben Ginsberg, attempted to bar moderators from offering any "comment" on "answers of the candidate."

Ginsberg says reporters should fact-check after the debate, not hijack the limited time during the debate for candidates to address each other.

Related: Clinton Hunkers Down at Hotel Ahead of Debate Showdown

"Those candidates are up there to fact-check each other," Ginsberg told NBC News. "This debate is not about the moderators or enhancing their ratings."

Memoranda of Understanding

In 2012, Crowley said she never saw or agreed to the campaigns’ "debate memorandum," which is basically a political agreement between competitors — not an alliance with the press or debate commission.

A debate memorandum is "the firm suggestion from the campaigns," Ginsberg said, "but there’s no enforceability, and neither the moderators nor the commission signed the document four years ago."

While the Debate Commission is not listed in the memoranda that have leaked, historically it has followed many of the campaigns’ terms.

The 2012 rules limited the topics, choreographed how the candidates were broadcast, and barred a wide array of potential debate features, including "opening statements," charts, "props," references to people "sitting in a debate audience," challenges by the candidates to make "pledges" and questions that "ask the candidates for a 'show of hands.'"

A spokesman for the Commission, which is run by leaders from both parties, declined to release or discuss this year’s rules when reached by NBC News.

On Monday, anonymous sources in both parties told NBC News that this year the two campaigns held talks about the debate structure, but did not reach a signed agreement. That is a contrast to past election years.

The Commission originally emerged from a controversy during the 1988 campaign.

The previous sponsor of debates, the League of Women Voters, refused to accept the mounting demands negotiated by the candidates. The group’s president objected to what she called "stringent, unyielding and self-serving demands" to constrain the debate.

The Commission stepped in as a sponsor, and has handled every general election debate since then.

Others have criticized the Commission for a bias against third-party candidates and a secretive approach to the process.

Open Debates, an group for reforming the debates, argues the Commission refuses "to release the Memorandums of Understanding negotiated by the major party campaigns" as an effort to "protect the major parties."

George Farah, who leads the group, says he is "sure that a Memorandum of Understanding has been written" by this year’s nominees.

"These are two candidates who operate under a tremendous amount of secrecy," he told NBC News. "This is the most secretive way the Commission has operated."

While it is normal for businesses to keep contracts confidential, the commission’s approach is at odds with a culture that increasingly expects transparency, especially from public institutions.

Ultimately, though, it is the campaigns that call the shots. They can officially release their negotiations and rules anytime. For all the talk about transparency this election cycle, neither campaign appears rushing to do so.