The Electoral College decides Hillary Clinton and Donald Trump’s presidential fate on Election Day, with a candidate needing 270 votes to secure the Presidency. The number is derived from the 538 electoral votes received from the nation’s 435 Representatives, 100 Senators and three District of Columbia electors.

During the 2012 Election, President Barack Obama defeated Mitt Romney by receiving 332 electoral votes compared to Romney’s 206. Clinton, who leads in nearly all general election polls, is polling below or in tight contents with Trump in battleground states like Florida, Ohio, New Hampshire and Iowa. Many outlets are predicting she may only come out with about 274 electoral votes, though still enough for her to claim the White House.

However, if Trump can sweep the swing states and come from behind to win states like Nevada and North Carolina, he would essentially win. The latest general-election polls show Trump narrowing in on Clinton, trailing by just 3.3 points, according to averages compiled by Real Clear Politics.

Should neither Trump nor Clinton gains the majority of the electoral votes needed, the election will be determined by three members of the House of Representatives. The candidate that receives the most House votes will be named the next president. The Senate, however, determines the vice-president.

While it is very uncommon for a president to win the popular vote and inevitably lose the election due to lack of electoral votes, it has happened before. In 2000, George W. Bush lost the popular vote by about 540,000 votes. However, he received 271 electoral votes compared to Al Gore’s 266 votes.

The first time a candidate won the presidency without the majority popular vote was during the 1824 election between John Quincy Adams and Andrew Jackson. Although Jackson technically won the popular vote by 38,000 votes and received 99 electoral votes compared to Adams’ 84, neither candidate reached the 131 Electoral College votes needed to win the election at that time, which lead to the House of Representatives’ decision to name Adams as the sixth president.

Similar instances happened during the 1876 election that led to Rutherford B. Hayes’ presidency and Benjamin Harrison’s win over Grover Cleveland during the 1888 election.