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The seventh Democratic presidential debate is set for Tuesday night, and it will be the smallest — and least diverse — debate to date.

The field of candidates has been shrinking, and the front-runners' campaigns have been taking on a tougher tone with one another, with even old friends Bernie Sanders and Elizabeth Warren butting heads.Follow live: Democrats debate in Iowa as first vote nears

The debate is the last before the Iowa caucus on Feb. 3. The two candidates who've been sniping at each other the most in recent weeks, former Vice President Joe Biden and Sanders, the independent senator from Vermont, will be standing side by side on center stage.

Here's what you need to know:

When and where is the debate?

The two-hour debate, which is being co-hosted by CNN and The Des Moines Register, is scheduled to begin at 9 p.m. ET at Drake University in Des Moines. The event has three moderators — CNN's Wolf Blitzer and Abby Phillip, and the Register's Brianne Pfannenstiel.

Who made the stage?

Higher qualification thresholds have quickly whittled down the field. To qualify for Tuesday's debate, candidates had to raise money from 225,000 unique donors and hit at least 5 percent in four different national polls or 7 percent in two polls from early voting states.

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The six candidates who qualified are Biden; Sanders; Warren, the senator from Massachusetts; Pete Buttigieg, the former mayor of South Bend, Indiana; Sen. Amy Klobuchar of Minnesota; and billionaire philanthropist Tom Steyer.

Sen. Cory Booker of New Jersey, who dropped out of the race on Monday, and entrepreneur Andrew Yang, both candidates of color, reached the donor threshold but not the polling threshold. Yang had been the lone candidate of color in December's debate.

Polling was used to determine where the participants will stand on stage, with the best-polling candidates in the center and the lower polling candidates fanning out to the sides. From left to right, the order is Steyer, Warren, Biden, Sanders, Buttigieg and Klobuchar.

What are the rules/format?

There will be no opening statements, but candidates will be able to make closing statements. Candidates will have 75 seconds second to answer questions, 45 seconds for rebuttals and 15 seconds for clarifications, debate organizers said Tuesday.

Where to watch the debate?

The debate will air live on CNN and CNN International and stream live on CNN.com and DesMoinesRegister.com.

NBCNews.com will live blog the debate, including fact checks and analysis.

When is the next round?

The once-a-month debate schedule changes in February, when three debates are scheduled for early voting states.

The first, to be hosted by ABC, WMUR and Apple News, is scheduled for Feb. 7 at St. Anselm College in Manchester, New Hampshire. The second, hosted by NBC and MSNBC, will be held in Las Vegas on Feb. 19. The third will be held son Feb. 25 in Charleston, South Carolina, and is being hosted by CBS News, the Congressional Black Caucus Institute and Twitter.

If President Donald Trump's impeachment trial is still going on at that point, it could cause severe scheduling problems for three of the six candidates in Tuesday's debate, since sitting senators will act as jurors in the trial.