Tomorrow is Election Day, when citizens of the United States go to the polls to elect a variety of public officials. This year is a midterm election (in contrast to both a Presidential election and “off-year” election years). Here are five facts you should know about midterm elections:

1. Midterm elections are the national elections in the U.S. that occur at the two-year midpoint of a president’s four-year term. Because members of the U.S. House of Representatives are elected for two-year terms and U.S. Senators for six-year terms, the all 435 House seats and one-third of Senate seats are decided at the midterm. Additionally, in 2018, the election will decide 36 state governorships and three U.S. territory governorships.

2. The party of the incumbent president tends to lose seats in Congress during midterm elections. Over the past century there have been 25 midterm elections. Of those, the incumbent president’s party has lost an average of 29 seats in the House and four seats in the Senate. The president’s party gained seats in both houses only two times: Franklin D. Roosevelt in 1934 and George W. Bush in 2002.

3. Warren Harding lost the most House seats (77 in 1922), and Franklin D. Roosevelt gained the most (9 in 1934). FDR also gained the most Senate seats (also 9 in 1934) while his former vice-president, Harry Truman, lost the most (10 in 1946). Only two presidents—Ronald Reagan in 1982 and Bill Clinton in 1998—neither gained nor lost seats in the Senate. (Every president has either gained or lost in the House.)

4. Since the 1840s, voter turn out for the midterm elections has dropped relative to the years for a presidential election. For example, according to Pew Research, 57.1 percent of the voting-age population cast ballots in 2008—the highest level in four decades. Two years later only 36.9 percent voted in the midterm election. But some researchers believe focusing on the voting-age population distorts the picture because so many people living in the country are ineligible to vote. For instance, roughly 20.5 million U.S. residents aged 18 and over (8.5 percent) of the voting-age population, were non-citizens, while another 3.2 million couldn’t vote because they were in prison or had been convicted of a felony. By subtracting those people, and adding in the 4.7 million American citizens living overseas but still eligible to vote, the population in 2012 that was eligible to vote was an estimated 222.3 million. Based on that adjusted base, turnout in recent elections was rather higher: 61.6 percent in 2008, 39.9 percent in 2010 and 58.2 percent in 2012.

5. In the early decades after the ratification of the U.S. Constitution, midterm elections typically drew more voters than presidential contests. At the time, most states only gave voting rights to property owners. Additionally, the federal government’s main power center was Congress rather than the presidency. As Pew notes, those conditions changed in the 1820s during theSecond Party System, when “most states repealed property qualifications, interest in politics soared as politicians increasingly appealed to ordinary people, and the parties directed much of their energy on capturing the White House after the disputed 1824 election (which John Quincy Adams won even though Andrew Jackson received the most votes).”