President Richard Nixon and President Bill Clinton were both subjected to impeachment proceedings.

If you’ve been on social media, or if you read any news today, you’re aware that an official impeachment inquiry against the President of the United States has begun.  But what is impeachment? How have impeachment inquiries played out in the past? What is the likely outcome of today’s inquiry? Here’s a quick guide, so you know what you’re talking about when you talk about impeachment.

What does impeachment really mean?  Per the Constitution, Congress can remove a president before his (or her) term is up, if enough lawmakers vote to say that the President committed “treason, bribery, or other high crimes and misdemeanors.” “High crimes”  generally refer to an abuse of power; it doesn’t necessarily mean that a crime has been committed. Jeffrey A. Engel, director of the Southern Methodist University Center for Presidential History and a contributor to the 2018 book Impeachment: An American History, tells Politifact that “A high crime is an affront to the state, to the people, the body politic. A president, or any leader really, need not break any statute in order to break the public’s trust.”

Have any past presidents been impeached? Only three presidents have been subjected to impeachment proceedings. Two of them were, in fact, impeached by the House of Representatives: Andrew Johnson in 1868 and Bill Clinton in 1998. However, they were both ultimately acquitted by the Senate and completed their terms in office.  Richard M. Nixon was the third, but he resigned in 1974 to avoid being impeached.

How does the impeachment process begin? According to the New York Times, this has been a subject of dispute. During the Nixon and Clinton impeachment efforts, the full House of Representatives voted for resolutions directing the House Judiciary Committee to open the inquiries. But it is not clear whether that step is necessary, because impeachment proceedings against other officials have begun at the committee level (Other positions beside the President who may be impeached include the Vice-President and federal judges). Regardless, impeachment begins in the House of Representatives, which debates and votes on whether to bring charges against the president via approval of an impeachment resolution, or “articles of impeachment,” by a simple majority of the House’s 435 members.  So, essentially they need 218 votes.

What if the House of Representatives votes to impeach? If the House approves such a resolution, a trial is then held in the Senate. House members act as the prosecutors; the senators as jurors; the chief justice of the U.S. Supreme Court presides. A two-thirds majority vote is required in the 100-member Senate to convict and remove a president. This has never happened.

What is the likely outcome? There are currently 235 Democrats in the House of Representatives, so a vote to impeach seems likely. However, the Senate now has 53 Republicans, 45 Democrats and two independents. Conviction and removal of a president would require 67 votes, so it seems unlikely that the President would be actually removed from office. If he was removed, Vice President Mike Pence would become president for the remainder of the current term, which ends on January 20, 2021.