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It’s been called the “red mirage” or the “blue shift.”

It’s the recent phenomenon by which an apparent Republican lead early after polls close on election night is erased by the counting of mail-in ballots later in the evening or in the days after Election Day.

Former President Donald Trump has pointed to the red mirage to back up his baseless allegations of election fraud, when in fact it has been a function of the rise of mail-in voting and the often-peculiar rules about when those ballots can be counted.

What happened in 2020?: When most Americans went to bed on election night in 2020, on November 3, the end result was far from clear. The contest between Trump and Joe Biden was still too early to call in the key states of Arizona, North Carolina, Nevada, Wisconsin, Michigan, Maine, Georgia and Pennsylvania.

The following days would include dramatic hours spent watching the counting of votes in these states. The vote count didn’t reflect a Biden lead in Georgia until early in the morning of November 6, when small batches of votes were being counted and affecting the very close election.

CNN was able to project that Biden would win the election four days after Election Day, on November 7, but counting would continue.

How long will it take this year?: There is some indication that things could go quicker in some key states.

In Georgia, a new election law means more people will vote early in person rather than by mail, which could speed up the process of counting mail-in ballots. North Carolina no longer accepts mail-in ballots if they arrive after Election Day.

It’s important to remember that while news networks like CNN might project a winner when it is clear who will win, the races are not officially certified until later. Certification deadlines vary by state, but they all have until December 11 to complete recounts, if needed, and to settle disputes around presidential results.

Read more about the “red mirage” or the “blue shift” here.