We are unlikely to know the winner of the Democratic primary race for mayor on primary night.

Polls show a close contest between two candidates, former Gov. Andrew M. Cuomo and Assemblyman Zohran Mamdani. In the ranked-choice election, voters can select up to five candidates in order of preference, and if neither man gets more than 50 percent of the first-place votes on Tuesday, a series of subsequent rounds will tally the final results based on voters’ second-through-fifth-place choices.

But that count will not take place until July 1, a week after the election, because absentee, mail-in and affidavit votes, which can be important in a close race, can be received and counted up until then.

Polls close in New York at 9 p.m., and first results will start to come shortly after that.

Here’s what else you’ll need to know ahead of Primary Day:

The math of ranked-choice voting

This is New York’s second mayoral primary election using the ranked-choice voting system. Vote counting proceeds in rounds, with the last-place candidate eliminated in each round. If a voter’s top choice is eliminated, the vote is then transferred to the voter’s next choice. Elimination rounds continue until there are two candidates left and one gets more than 50 percent of the vote.

Most reliable polls suggest that neither Mr. Mamdani nor Mr. Cuomo will receive more than 50 percent of the vote in the first round of vote counting on Tuesday night. But their performances will offer a look at who has the upper hand: The closer a candidate is to 50 percent, the better chance that candidate has to win in the end.