Congress formally opened a new, public phase of its presidential investigation on Thursday as US lawmakers voted for the first time to advance the impeachment process against Donald Trump."Today, the House takes the next step forward as we establish the procedures for open hearings conducted by the House Intelligence Committee so that the public can see the facts for themselves," House Speaker Nancy Pelosi said shortly before the vote.The chamber voted largely along party lines, 232 to 196, to formalise the process, which also provides for opportunities for Trump's counsel to cross-examine witnesses.Six things to know about the impeachmentAmid controversy over a summer phone call between US President Donald Trump and the new leader of Ukraine, Pelosi ordered a formal impeachment inquiry of the US president in September. "No one is above the law," she said as she announced the House was moving forward with an official impeachment inquiry of Trump. Pelosi, a Democrat, had earlier resisted calls to begin impeachment proceedings, urging restraint as House committees aggressively investigated Trump, a Republican, through subpoenas of witnesses and documents.The Greatest Witch Hunt In American History!— Donald J. Trump (@realDonaldTrump) October 31, 2019 But following allegations that Trump pressured the leader of Ukraine to investigate the family of former Vice President Joe Biden, who is vying for the 2020 Democratic presidential nomination, pressure from inside the Democratic caucus for an impeachment inquiry mounted. Since then, three House committees have held closed-door hearings, deposing current and formal officials who were close to the dealings at the centre of the inquiry. The Impeachment Hoax is hurting our Stock Market. The Do Nothing Democrats don’t care!— Donald J. Trump (@realDonaldTrump) October 31, 2019 The House on Thursday is expected to approve ground rules for moving forward with public hearings. Here is what you need to know about the US impeachment:1. What is impeachment in the US political system?The founders of the United States included impeachment in the US Constitution as an option for removal of presidents by Congress. Impeachment, a concept in English common law, was one of the more hotly debated points during the constitutional convention of 1787 in Philadelphia. Delegates agreed that presidents could be removed if found guilty by Congress of "treason, bribery or other high crimes and misdemeanors".The sole authority under the Constitution to bring articles of impeachment is vested in the House of Representatives where proceedings can begin in the Judiciary Committee. If the House approves articles of impeachment, or "impeaches" a president, he or she would then be subject to trial in the US Senate.2. On what grounds can a president be impeached? How does impeachment work?Under the Constitution, the president, vice president and "all civil officers of the United States" can be removed from office for "treason, bribery, or other high crimes and misdemeanors".To begin impeachment proceedings, a House member can introduce an impeachment resolution, or the entire House can vote to initiate an investigation into whether there are grounds for impeachment. The House Judiciary Committee or a special committee will then investigate. The panel votes on whether to bring a vote to the full House. Impeachment in the 435-member House must be approved with a simple majority. If the House votes to impeach, the matter moves to the Senate, where a trial is held. The chief justice of the US Supreme Court presides over the trial.A two-thirds majority vote is required in the 100-member Senate to convict and remove a president from office.The Senate is made up of 53 Republicans, 45 Democrats and two independents who caucus with the Democrats. At least 20 Republicans would have to vote with all Democrats and the two independents to remove the president.3. Which presidents have been impeached?Only two US presidents have ever been successfully impeached and in neither instance was the president removed from office. Andrew Johnson was impeached in 1868 in the tumultuous aftermath of the American Civil War; and Bill Clinton in 1998 for issues including his relationship with White House intern Monica Lewinsky. Both times, the House approved formal charges and impeached the president, only to have the Senate fail to convict and remove him.The House Judiciary Committee in 1974 voted to recommend impeachment accusing another president, Richard Nixon, of planning to obstruct an investigation in the Watergate scandal. Before the full House could vote on impeachment, Nixon became the only US president ever to resign.4. Who would become president if Trump was impeached and removed? What would happen to Trump?A Senate conviction that removed Trump from office would automatically elevate Vice President Mike Pence to become president, completing Trump's term, which ends on January 20, 2021. Criminal charges cannot be brought against a sitting president, however, the Constitution does allow for separate criminal charges once a president is removed.5. What's at the centre of the Ukraine phone call? Despite her early reluctance, Pelosi's position became untenable as more members - including crucial moderates in political swing districts - swung in favour of an impeachment inquiry following reports that Trump pushed Ukrainian President Volodymyr Zelensky for help investigating Democrat Biden and his son during a summer phone call.The reports centred on a whistle-blower complaint that reportedly includes information on the phone call, as well as other events. Trump has sought to implicate Biden and his son in the kind of corruption that has long plagued Ukraine. Hunter Biden served on the board of a Ukrainian gas company at the same time his father was leading the Obama administration's diplomatic dealings with Kyiv. Though the timing raised concerns among anti-corruption advocates, there has been no evidence of wrongdoing by either the former vice president or his son.Zelensky agreed to Trump's requests. The aid was later provided. Trump has suggested that he brought up Biden and his son Hunter in the phone call as part of discussions over corruption in Ukraine. He also confirmed that he ordered advisers to freeze $400m in military aid to Ukraine in the days before the phone call, prompting Democrats to charge that he was holding out the money as leverage for information on Biden.6. What does Trump say?Earlier in the inquiry, Trump asserted that a formal impeachment inquiry by House Democrats would be "positive for me". He said the country is "doing the best it's ever done" and that Democrats are going to lose next year's elections if they pursue impeachment. But following the announcement, he lashed out at Democrats, calling the official inquiry "witch-hunt garbage". "Such an important day at the United Nations, so much work and so much success, and the Democrats purposely had to ruin and demean it with more breaking news Witch Hunt garbage," Trump said on Twitter, referring to the UN General Assembly (UNGA) last month. "Can you believe this?"Trump has also called on Republicans to come to his defence. He maintains that he did nothing wrong, and that the call between him and the Ukrainian leader was "perfect".