Californians will head to the polls on June 2 to vote in the state’s widely anticipated primary election. 

At the heart of the primary is the tumultuous race to replace Gov. Gavin Newsom, who can’t run for his office again because of term limits. The race, which has been filled with twists and turns, has drawn national attention. In April, then-Rep. Eric Swalwell, who was considered to be the frontrunner in the crowded field of Democratic candidates, suspended his campaign after several allegations of sexual misconduct surfaced. Swalwell, who denied the allegations, also resigned from Congress. 

Swalwell’s exit from the gubernatorial race sparked speculation about which of the other Democratic candidates would emerge as the most likely figure to replace Newsom. Recent polls show that Tom Steyer, a billionaire philanthropist who previously launched an unsuccessful 2020 presidential bid, and Xavier Becerra, former Health and Human Services Secretary, are the leading party candidates, but it’s unclear which one will come out on top.

Polls also indicate that a Trump-backed Republican candidate, Steve Hilton, is another leading contender in the primary race. Hilton, who previously served as a political strategist for former British Prime Minister David Cameron, is among the two Republicans vying for Newsom’s role.

The race has triggered some concerns among Democrats that the party could be locked out of the general election in November if the vote is split between the large number of Democratic candidates, though experts have told TIME that that possibility is unlikely. But that outcome is possible because the state has what’s commonly called a “jungle primary,” or a “top-two primary” system.

Here’s what to know about California’s primary system.

What is a top-two primary?

In a top-two primary system, whichever two candidates receive the most votes advance to the general election—regardless of their party affiliation. That means that, hypothetically, California voters could only see two Democratic candidates on the November ballot—or only two Republican candidates, though the latter is less likely in the historically blue state.

Which races does the system apply to?

The system applies to California’s state constitutional offices, state legislative offices, and U.S. congressional offices—known as “voter-nominated offices.” But the top-two primary system doesn’t apply in the state for candidates who are launching bids for President, county central committees, or local offices.

How long has California used the top-two primary system?

The Top Two Candidates Open Primary Act, which established the so-called “jungle primary” system, went into effect in 2011, after voters passed Proposition 14 in 2010 approving the change. Supporters of the change said that top-two primaries would encourage people running for office to reach voters across the political aisle, thereby narrowing the partisan divide in the state. But others argued that it would restrict the options available to voters in a general election, should only candidates from the same party make it on the November ballot.

How do write-in candidates work in a top-two primary?

Write-in candidates can run for voter-nominated offices in a primary, but can only advance to the November election if they are one of the two candidates to earn the most votes in the primary.