Things could get very, very weird on election night.

After polls close at 8 p.m. in Pennsylvania, the first results that come in could make it look as if former Vice President Joe Biden is winning in a landslide. But then the numbers could start to shift, and by the end of the night, President Donald Trump could look as if he’s the one winning big. Then, in the days afterward, vote tallies could slowly turn back in Biden’s favor.

All this means we may or may not know who actually won Pennsylvania — or the White House — on Nov. 3. It could be a jarring change for Americans who have long since become accustomed to going to bed knowing the winner.

Don’t panic. Here’s how to understand what we will and won’t know on Election Day, and why.

The good news: We’ll have a lot of information

Winning the presidency requires winning enough states to get to 270 Electoral College votes. So while Pennsylvania and its 20 electoral votes are key, we’ve got a whole country full of data to look at.

States generally move together, so the results from other states will help us understand Pennsylvania’s early results, said Charles Stewart III, a political science professor at MIT and a national expert on election results.

After months of anxiety about the potential for a days-long wait, Stewart thinks we might know the winner on election night, after all — or at least the path to get there.

“We’ll know the path to 270 by midnight, I’m pretty confident, and I can even see the AP calling enough states for 270 by the time the sun comes up,” he said. But, he added, that depends on states like Florida showing a clear winner on election night.

Consider Florida, North Carolina, or Kentucky, Stewart said: Their votes will be counted very quickly, and if the states shift several points toward Biden compared with Hillary Clinton’s 2016 vote — especially in key counties that closely mirror parts of Pennsylvania — we’ll have a good sense of how the election is going.

So even if Pennsylvania doesn’t count its votes quickly, other states do, and no state’s election is taking place in a vacuum.

The results from other states will also help show the difference between the mail and in-person vote. Some Pennsylvania counties, particularly the smaller ones, will likely count their mail ballots quickly. We’ll also know how many mail ballots were requested overall, including from which parties and counties, and we might even have a good sense of how many have been returned and not yet counted.

The bad news: There are a lot of unknowns

There are some key questions we won’t have the answers to until votes start getting counted, especially if the election is close — as it was in 2016, when Trump defeated Hillary Clinton in Pennsylvania by less than 1% of the total votes cast.

Here are some key things we won’t know:

  1. How many mail ballots will arrive after Election Day? The state Supreme Court has said those ballots can be counted if they are postmarked by Election Day or have missing or illegible postmarks and arrive by 8 p.m. that Friday.
  2. How many mail ballots will be rejected? This could include ballots that are unsigned, or “naked ballots” missing inner secrecy envelopes.
  3. How will turnout differ across areas? What matters isn’t just the two-party vote share but the actual level of voter turnout. We know that deep-blue cities such as Philadelphia will vote for Biden, and deep-red rural areas will go for Trump. But it’s the raw numbers that add up to a margin of victory or defeat.

And counting could be slowed down, for example, if there are court challenges. We just don’t know yet what that looks like.

It will take days to count the votes

It’s the first year any Pennsylvania voter can vote by mail, and the pandemic has fueled a massive surge in demand for mail ballots. Those ballots take time to count, and the process, by law, can’t begin until the morning of Election Day. Meanwhile, Trump has spent months falsely attacking mail ballots as susceptible to widespread fraud. The result has been a strong partisan divide: Democrats are disproportionately likely to vote by mail while Republicans are voting in person.

What we’re used to seeing hour-by-hour could happen day-by-day, said Ned Foley, an Ohio State election law professor.

Votes are counted in a specific order

There are three different methods for casting a vote, and that affects when they are counted.

  1. The first ballots to be counted will be the mail ballots that were returned early on. These will be among the first votes reported after the polls close, which is why things could look very good for Biden at first.
  2. Then the in-person votes get counted. Because those votes are being put into electronic machines, getting their results is easy — all the votes from a machine can be added to the county’s tally within minutes. Those results will likely skew heavily toward Trump.
  3. Then the rest of the mail ballots get counted, which will take days. Elections officials have to check the voter information on the outside envelope, open that outer envelope, pull out the inside envelope, open the inside envelope, pull out the ballot inside, unfold and flatten the ballot, and scan the ballot to count it.
  4. And finally, the last ballots to be counted are the provisional ballots, the paper ballots used at polling places when it’s unclear whether a vote should be counted.

As these mail and provisional ballots are counted, they will likely continue what is known academically as a “blue shift” toward Biden. That’s not fraud, or the election being stolen, it’s just the votes being counted. How big that shift is could determine the winner.

We don’t know how quickly those ballots will be counted, and estimates have changed over time. Counties have spent millions of dollars buying new equipment to speed up the counting and have significantly increased their staffing, with Philadelphia and its suburban counties planning to count ballots around the clock.

Chester County officials said they plan to have all the ballots they have received counted by sunrise Nov. 4. Delaware County hopes to complete that process by 1 a.m. for votes received before Election Day, and then to have all the ones received on Election Day counted by 8 a.m. Thursday.

But then ballots can keep arriving until Friday.

How races are ‘called’

When a race is “called” on election night or soon after, it’s not an official declaration from elections officials — it’s a determination made by news outlets that enough data have come in to know who won.

Those calls aren’t always right, including the infamous “Dewey Defeats Truman” headline in the Chicago Tribune in 1948, or the TV news declarations that Al Gore had won the 2000 election. There have been more recent examples, including when the Associated Press — which The Inquirer uses as its source for declaring winners — called a California congressional race in 2018 before later retracting the call.

Still, the AP and other news organizations that call races have a strong record of accuracy, using sophisticated statistical models and a variety of data sources paired with on-the-ground reporting.

So what can we expect?

Remember: There’s a difference between knowing the final vote count and knowing enough to unofficially declare a winner.

How quickly news organizations like the AP can call the race depends on how close the margins are and how things play out in the rest of the country.

There very well may be enough information on election night to call the presidential race, especially if it’s a landslide.

But there’s also a real chance that it might take a day or two, though it shouldn’t take longer than the end of the week. If it does, it will likely be because something has gone wrong in the counting process or because litigation throws things into doubt.

A ton of votes will be counted on election night, and counties will be scrambling to count them all as quickly as possible.

As they do, the best course of action is patience.

The ultimate goal is to count every vote and have confidence in the system.

“However long it takes after Nov. 3, however long it does, that’s a good thing, because that speaks to accuracy," Foley said. "It speaks to legitimacy. It speaks to all of the values that allow us, the voters, to control our destiny. It means we are getting the result that we want. We just may not be getting it as soon as we want.”