Once the last voter casts their ballot in a state and the polls close, the process of revealing the winner begins. Then election day turns into election night and each state begins reporting their vote totals.

Some states – such as Florida, Georgia and North Carolina – report their vote quickly, while others, such as Arizona, Nevada and California, usually take longer, more than a week or two, to- and recorded most of the ballots. In many states, the patterns of how votes are reported can make it difficult to tell in the middle of election night who the eventual winner will be.

For example, votes may tilt toward one party early on a night because only a certain one type the vote is reported first – as when mail-in ballots are counted before in-person votes on Election Day. These patterns can create what are sometimes called “voting mirages.”

A the mirage of the vote is when the current vote count shows that a candidate is getting a higher percentage of the vote than they will eventually you reach the final count. Mirages can be “red” or “blue” depending on which party appears to benefit – but “seems” is the really important thing here because, like any other mirage, a voting mirage is fleeting and does not reflect ultimate reality.

The factors that produce a voting mirage

In general, there are three factors that produce voting mirages: geography, voting method, and tabulation order.

Mirages may occur due to geographic differences between counties that report faster or slower. It’s common for small rural counties or precincts to report their tallies faster than large, urban ones because the logistics of counting votes are simpler when there are fewer ballots to handle. As a result, since rural voters are more likely to be Republican than urban voters, a red mirage can occur when a state’s vote count overrepresents rural areas at some point on election night.

Virginia is an example of a state that typically has a geography-driven red mirage early on election night. That’s because Fairfax County, a populous suburb of Washington, DC where Joe Biden received 70 percent of the vote in 2020, tends to be one of the last counties in the state to report most of its results. Until Fairfax County reports most or all of its votes, Donald Trump will likely have a percentage of the statewide vote that is higher than his final statewide total.

On Election Night 2020, Fairfax County reported the results of most of its ballots (nearly 375,000 votes) at 11:43 PM ET. This one-time ratio caused Trump’s statewide vote share to drop from 50.2% to 45.8% at the time. (That’s close to Trump’s final vote in Virginia, 44%).

Another factor that can cause voting mirages is partisan differences based on how to vote – mail-in ballots, early in-person voting, in-person voting on Election Dayand so on.

Many states and counties tend to report the results of each mode in turn. This can cause voting mirages if one party is more likely than the other to cast their vote using a particular mode. That was the case in 2020, when Democrats voted more by mail than Republicans, and Republicans were much more likely to show up on Election Day.

This difference caused a big blue mirage in North Carolina in 2020. Within the first hour after the polls closed, most North Carolina counties reported their mail-in and early in-person votes. Those votes were heavily Democratic, and as of 8:18 p.m. ET, Biden was at 57.6 percent of the statewide vote. But over the next three hours, the state reported its heavily Republican vote on Election Day, causing Biden’s share of the vote to drop to his final tally there of 48.6 percent.

The third factor that can produce voting mirages is more complicated: differences in the order in which certain ways of voting are counted. This tends to be the most important in states and counties that report mail-in ballot results in small lots.

Many election offices tend to count their mail-in ballots in the order in which they were received, meaning that ballots returned by voters in mid-October will appear on the ballot earlier than those received on Election Day. This will produce a voting mirage if there are partisan differences between mail-in ballots returned earlier and those returned later.

This is what happened in Arizona in 2020. Mail-in ballots reported by Maricopa County (which represents 60 percent of the state’s voters) on election night included those returned by voters through the weekend before Election Day. This ended up producing a blue mirage because those earlier postal votes were more democratic than the later ones.

The results of mail-in ballots returned later were reported in the days following Election Day. Because those mail-in ballots that arrived late were heavily pro-Republican, Donald Trump’s vote count rose from 46.8 percent at 3 a.m. ET on election night to his final vote share of 48.9 percent – just below Biden’s 49.2%.

Interestingly, Arizona’s pattern in the 2018 midterm elections was the opposite. Mail-in ballots that arrived late were much more heavily pro-Democrat, and once they were added, Democrat Kyrsten Sinema went from down 0.8 percentage points on election night to winning the election by 2 points.

What to look for in terms of mirages in each state

The NBC News decision office takes these mirages into account when deciding whether to project the winner in a race. For example, we are less likely to project a Republican winner in a state if all the remaining uncounted ballots are expected to come from heavily Democratic counties. Conversely, we are less likely to make a projection if the Democrat is in the lead, but the majority of uncounted ballots are expected to be heavily Republican in person on Election Day.

We’re also cautious about assuming historical patterns will hold in 2024, especially given that the 2020 election took place during a pandemic, with voters more likely to vote by mail and social distancing affecting how officials electoral officials tabulated the results. This year, Trump’s campaign also encouraged supporters to vote early, which can affect partisan differences in voting patterns.

Here are specifics about the mirages we’ve seen in key swing states in the past — and what could happen on election night this year.

North Carolina: Most of the mail-in ballots will likely be reported first, causing a blue mirage. In a change for this year’s election, early in-person ballots cannot be tabulated until after polls close on Election Day. We expect in-person early voting to begin around 8:00 PM ET, half an hour after polls close in the state. But vote share will shift toward Republicans as in-person Election Day votes are reported throughout the evening.

Georgia: Most counties will report their early in-person and mail-in ballots first, likely causing a blue mirage before statewide results swing toward Trump as Election Day votes are reported. After that, there may be a small shift back to Democrats as late-arriving mail-in ballots are reported from large Atlanta counties. Unlike in 2020, Georgia counties are not allowed to stop the vote counting process until they have counted all the ballots.

Pennsylvania: We expect the first reported votes to be dominated by mail-in ballots, including in heavily Democratic Philadelphia and Allegheny counties, causing an early blue mirage. Then, as in-person votes are reported on Election Day, the blue mirage will turn red, with the results going more Republican than we expect to see in the final margin. Finally, those large counties (especially Philadelphia) will continue to count and report the remaining ballots. This will likely swing the race back to the Democrats.

In 2020, Pennsylvania results at 3 a.m. ET on election night were about 7 percentage points more Republican than the final result. In the 2022 Senate election, the red mirage at 3am was just over 1 percentage point short of the final margin.

Michigan: We expect a slight red mirage in Michigan as smaller, more Republican-leaning localities report their votes. Vote totals will likely increase as small places report, and the size of the mirage will shrink when large, Democratic cities like Detroit, Grand Rapids and Ann Arbor report. This year, Michigan voters can vote early in person for the first time in a general election, and clerks are now allowed to pre-process mail-in ballots, allowing for a faster count. Both of these changes may cause differences in vote reporting patterns compared to what we’ve seen in previous elections.

Wisconsin: In most Wisconsin municipalities, all voting methods are reported together. This produces a slight red mirage as smaller, rural municipalities tend to report faster.

A handful of large municipalities, including the city of Milwaukee, tabulate their mail-in ballots centrally and report them separately from their Election Day votes. The state will experience a red mirage until heavily Democratic mail-in ballots are reported from most heavily Democratic cities in the state. At 4:42 a.m. ET on Election Night 2020, Milwaukee reported that Biden had received 85 percent of the 168,000 mail-in ballots. That pushed Biden’s statewide total up from 47.3% to 49.3%.

Arizona: In 2020, about 70 percent of the state’s votes — mostly in-person and early mail-in ballots — were reported within the first hour after the polls closed. That gave Biden a statewide lead of more than 14 percentage points in a state he ultimately won by 0.3 points.

As Election Day votes were reported over the next few hours of Election Night, the size of the blue mirage shrank to about 3.5 points. It continued to shrink after Election Day as pro-Trump mail-in ballots were reported arriving late. In the 2022 Senate race, the Democrat was ahead in the first hour by 19 points and won by 5. But in the 2018 Senate race, the pattern worked in the opposite direction, with the Republican ahead by 3 points before losing in the latter with 2. points.

Nevada: Nevada will experience a red mirage until Clark County (Las Vegas) reports. In 2020, Clark’s first report came 90 minutes after the state’s first county reported its votes. In 2022, this gap was only 10 minutes. Then the first report from Clark may cause a blue mirage that will shrink as in-person votes on Election Day are tabulated statewide.

Mail-in ballots that arrive late will be reported in the days following Election Day. In 2020, these ballots moved Biden’s lead from 0.6 percentage points to a final result of 2.4 points.

This story first appeared on NBCNews.com. Read more from NBC News here: