A primer on the Electoral College and the roles of electors:

What is it?

It’s the 538 Americans who actually elect the president. The number corresponds to the seats a state has in the U.S. Senate and the House of Representatives, plus the three electoral votes allotted to Washington, D.C. The magic number is 270, the bare majority required to win the presidency.

Electors meet in their state capitals to cast their ballots; this year, that will be on Dec. 19.

How can a candidate win the popular vote and not the Electoral College?

The national popular vote simply doesn’t matter. Electoral votes are instead awarded based on state-by-state results. Essentially, the U.S. holds 50 separate popular votes that determine an Electoral College vote count.

How many times has a candidate won the popular vote but lost the election?

According to the U.S. House: Andrew Jackson in 1824; Samuel Tilden in 1876; Grover Cleveland in 1888; Al Gore in 2000; and Hillary Clinton in 2016.

How did the college originate?

The Electoral College is an original feature of the Constitution. Some framers wanted popular election of the commander in chief. Many others didn’t trust the masses. The Electoral College was a compromise. Pegging a state’s electoral vote count to the size of its congressional delegation was a negotiated design, giving small states more influence proportionally than large states.

Do electors have to vote the way their state voted?

The Constitution is silent on this point, which suggests electors can go their own way. This is certainly the thinking behind petitions and a handful of Clinton electors urging Republican electors to abandon Trump.

In 29 states, there are either statutes or party rules that theoretically bind electors to honor state results. But the penalty for becoming a “faithless elector” is typically a fine measured in the hundreds of dollars. The U.S. Supreme Court has never ruled explicitly on those state laws and party rules, and some constitutional scholars say such state restrictions would be struck down were they ever challenged.

Have there been “faithless electors” before?

The National Archives says that throughout the nation’s history more than 99 percent of electors have voted as pledged.

According to research by FairVote.org, a nonprofit group that advocates national popular vote elections for president, there have been just 157 “faithless electors.” That represents less than 1 percent of the total electoral votes cast in the nation’s history.

Only a handful of those have been in the modern era. The last publicly identified faithless elector was Barbara Lett-Simmons of Washington, D.C., in 2000.

A Gore elector, Lett-Simmons abstained as a public protest for her home city having no voting representation in Congress. Lett-Simmons reportedly sought, and received, Gore’s permission for her act of defiance.

What happens if no candidate receives the 270-vote majority?

Under the Constitution, the House of Representatives decides, with each state delegation casting a single vote for one of the top three vote-getters. Republicans control a majority of state delegations, so this route still benefits Trump.

The House has decided a presidential election only twice, in 1800 and 1824.