Republican U.S. presidential candidates pose during a photo opportunity before the debate held by Fox Business Network in Milwaukee, Nov. 10, 2015. Reuters/Darren Hauck

The Republican presidential candidates will face off Tuesday night for the last time in 2015. CNN is hosting the debate with help from Facebook, and the event will take place in Las Vegas.

This is the first debate to use early-state polls as a criterion for being included in the main event. To be included, candidates must have met one of three criteria in CNN-recognized polls conducted between Oct. 29 and Sunday: an average of at least 3.5 percent support nationally; at least 4 percent in Iowa; or at least 4 percent in New Hampshire. These criteria saved Kentucky Sen. Rand Paul, who was in danger of being relegated to the undercard debate, along with New Jersey Gov. Chris Christie, Carly Fiorina and Ohio Gov. John Kasich.

Donald Trump maintained his lead in the polls despite his month of controversies since the last GOP debate, so he will take center stage during the prime-time event. Ben Carson will stand to Trump’s right, and Texas Sen. Ted Cruz, who has recently shot up in the polls, will be on Trump’s left. Also on the main debate stage will be Florida Sen. Marco Rubio, former Florida Gov. Jeb Bush, Fiorina, Christie, Kasich and Paul.

Before the main debate at 8:30 p.m. EST, CNN is hosting an undercard debate at 6 p.m. EST. The junior varsity event will comprise former Arkansas Gov. Mike Huckabee, former Pennsylvania Sen. Rick Santorum, South Carolina Sen. Lindsey Graham and former New York Gov. George Pataki.

RCP Poll Average for Republican Presidential Candidates | InsideGov

As the final debate of the year, Tuesday’s showdown is likely to be seen as an important way for candidates to leave voters with a positive image over the holidays before they ramp up their campaigning efforts in January to prepare for the first primary contests. Below are the rules for Tuesday night’s debate.

Supplies: As in previous debates, the 13 candidates won’t be allowed to bring much with them to the debate stage. They can’t have notes, props or cell phones at the lecterns. But the candidates are typically given a glass of water along with a blank notepad and a pen, and that should remain true Tuesday night.

Questions: Wolf Blitzer will be moderating the debate, with CNN chief political correspondent Dana Bash and Salem Radio Network talk show host Hugh Hewitt asking the questions. Because CNN is partnering with Facebook, viewers can ask questions on the social network and some of them may be included in the debate.

Subjects: This will be the first GOP debate since the terror attacks in Paris last month, so the debate is likely to have a heavy focus on national security issues. With Blitzer known for his foreign policy chops, a CNN source told Politico that candidates should think of this as the “commander in chief” debate. In CNN’s Facebook post asking for audience questions, the media company offered suggested topics, including “gun control, terrorism or how to keep America safe.”

Answer time: In the past, candidates have complained about the short time allowed for answering questions, and many have ignored moderators who tried to stick to the rules. CNN has not said how many seconds each candidate will have to answer questions or rebut others, but there will surely be time constraints Tuesday night.