If President Clinton is impeached this week he will face trial in the Senate which last tried a president in 1868. If the trial takes place next year it could be a spectacular affair, with Monica Lewinsky and Linda Tripp among the witnesses called.
The 26 rules for the trial of President Andrew Johnson 130 years ago are still in force and were reviewed in 1974 in case they were needed for President Richard Nixon, who resigned before he was impeached.
The Senate trial would be presided over by Chief Justice William Rehnquist and not the president of the Senate, who is Vice-President Al Gore.
The chief prosecutor would probably be Mr Henry Hyde who chaired the Judiciary Committee of the House of Representatives which voted the four articles of impeachment.
President Clinton could appear in person, if he wished, with his defence team. President Johnson never appeared for his trial, in which he was acquitted by one vote.
The 100 senators would be the jury, but they would not be allowed to speak apart from their private deliberations before giving their guilty or not-guilty verdict as they stand by their chairs.
During the trial, the senators could pass questions for witnesses in writing to the Chief Justice, who would pose the questions.
A conviction on the basis of the articles of impeachment sent from the House of Representatives would require a two-thirds majority or 66 votes. At present the Republicans have 55 votes to the Democrats' 45, but some senators are now saying that if a trial does take place nothing should be taken for granted about the outcome. It could take a momentum of its own, and some Democratic senators could vote against the President.
If impeachment does take place and a trial begins some time next year, the Senate has the power to halt it at any time by a simple majority. This power could be used to allow the Senate to opt for a censure resolution which could be agreed with President Clinton as an alternative to the trial. This has been suggested by the former presidential candidate, Mr Bob Dole, who is also a former Senate majority Leader.
Another unknown factor is how long a trial could take. The President would have to be given several weeks if necessary to prepare his defence. The Senate would also have to hear motions defining its rules of evidence and what is meant by perjury as it would figure in the articles of impeachment.
One possibility is that the Senate would conduct the trial in the mornings and do its other business in the afternoon. Apart from the trial of President Johnson, which took several months, the Senate has carried out 12 other impeachment trials, mainly of federal judges, and has carried on other business at the same time.
It is likely that the Democrats in the Senate may at the beginning challenge the validity of an impeachment resolution coming from the outgoing 105th House of Representatives and demand that the new House elected last November vote again on impeachment. The Chief Justice would be asked to rule on this matter.