If you’re tuning into Election Day in the United States now, voting in the midterm elections ends when the polls close on Tuesday. Millions of Americans have already voted either early, in person or by mail, and millions more will vote in person at their polling places. Here’s a primer on the 2022 election to bring you up to speed as the country waits to find out who will be in charge of Congress in January.

WHICH MATCHES ARE ON THE BALLOT?

Every US House seat is up for election this year, along with about a third of the US Senate. On the line is control of both houses of Congress, currently under Democratic leadership. Voters are also electing governors in most states this year. They will take office in 2024, when the next presidential election is held, and could affect election laws or vote certifications. Also on the ballot are many state legislative and local officials.

HOW DOES MEASUREMENT WORK?

The United States has a highly decentralized electoral system, with local officials responsible for conducting the voting and counting the results, while the states certify the results. Not all states vote the same: Some run only mail-in voting, while others have a mix of in-person and mail-in voting. Others also have early in-person voting periods.

HOW LONG CAN IT LAST?

There is no federal government agency that tells the country who won the election immediately, and different states count ballots at different times. This means that it may take longer to declare a winner in some places. Different rules about when recounts or repeat elections may be required could also have an impact. Some states like Pennsylvania and Wisconsin, for example, don’t allow officials to begin validating mail-in ballots until Election Day. Other states allow grace periods for counting ballots as long as they were sealed after Election Day. All of this means we likely won’t know who won each race come election night. The Associated Press calls races when it determines a clear winner. But no state announces full and final results on election night, nor has it ever in modern history, according to experts.

WHAT ARE THE POSSIBLE OUTCOMES AND WHAT WILL THIS MEAN?

Democrats and Republicans could split control of the House and Senate, or one party could take both houses. Democrats have held the House since the 2018 election during the administration of former President Donald Trump. Buoyed by voter frustration with the economy and favorable redistricting, Republicans are favored to win the House. A Republican-led chamber would mean President Joe Biden’s agenda would grind to a halt. The Senate, which Democrats hold the smallest advantage, could go either way. At stake is control over the promotion of presidential appointees, such as judges and cabinet officials, as well as legislation. As chief executive, Biden will retain his veto power regardless of how congressional scrutiny plays out.