Super Tuesday is one of the biggest milestones in a U.S. election year. But if you’re confused about how the U.S. primary system works, you’re not alone. The system isn’t simple. Here are six things to know about how Super Tuesday works.

What are people voting for?

In the United States, the Democratic and Republican parties both choose their presidential candidates at party conventions in the summer where delegates vote.

During the primaries, voters in each state cast ballots for a candidate. Those votes then translate into delegates who will go on to choose the candidate at the party’s primary convention.

Who votes?

In some states, there are open primaries where any registered voter can cast a secret ballot in a primary race. Other states hold closed primaries, where only voters who belong to a party are allowed to cast a ballot in that party’s primary contest. Most states that hold open primaries don’t allow voters to take part in votes for both parties.

Other states yet hold caucuses, where registered party members in state districts get together to openly decide which delegates they will send to the convention and who they will vote for.

In some contests, voters directly choose delegates, while in others, the party assigns delegates friendly to a particular candidate based on the outcome of the primary vote.

Why do they call it “Super Tuesday”

State primary contests are held over a number of months from February to June.

Super Tuesday is the date in February or March on which more state primary contests  are held than on any other single day. That means more delegates will be up for grabs on this day than on any other single day in the primary process.

Why does it matter?

While a party may have a crowded field of candidates going into Super Tuesday, the outcome of the day usually gives a good indication of who stands a serious chance of becoming the presidential candidate.

Following Super Tuesday, a party may coalesce around one or two candidates or a candidate who has a poor showing may decide to pull out of the race.

What does Super Tuesday mean for the Democrats?

On the Democratic side, 865 delegates will be up for grabs in 11 states and American Samoa. Hillary Clinton is going into Super Tuesday with momentum over her rival Bernie Sanders. Clinton has won three of four early contests and is believed to be favoured among Black voters in many of the states up for grabs today.

Super Tuesday could mark a pull-away moment where she firmly takes the lead or it could show that the feisty Sanders remains a stronger challenger than some have thought.

What does Super Tuesday mean for the Republicans?

The Republican party started off with a crowded field of candidates this year. Five remain after the early primary contests.

There are 595 delegates up for grabs in 11 states. Billionaire Donald Trump leads the pack, having won three out of the four early contests. His nearest rivals are Texas Sen. Ted Cruz and Florida Sen. Marco Rubio.

Trump has defied expectations with his success in the race, but has also deeply worried some parts of the Republican Party who believe that some of his outlandish pronouncements about immigrants and others might make him unelectable in the general election.

If Trump emerges from Super Tuesday as the big winner, the Republican Party will have to come to terms with what his candidacy means for the party.

-With files from AP