From the time a trial is opened, the Chief Justice of the United States presides in the Senate chamber. The present Chief Justice, William Rehnquist, 72, is a specialist in constitutional law and has made a special study of impeachment. He is held in high regard by fellow lawyers but also in awe as a stickler for procedure - and quiet.
There was some confusion about how he should be addressed, with Mr President, Mr Presiding Officer, and Your Honour mooted. It was decided he should be addressed as Mr Chief Justice. One of the rules altered for the Bill Clinton trial is the timetable. On most days proceedings are to start at 1pm rather than noon, to accommodate Mr Rehnquist's Supreme Court commitments. He has also been allocated an office at the Senate, equipped, for the first time, with a telephone.
The prosecution is represented by the 13 "managers" appointed by the House of Representatives, who will sit on the left of the chamber. All are Republicans, all are lawyers and all are on the Judiciary Committee, which formulated articles of impeachment against Mr Clinton in the House. They are led by Henry Hyde, who chaired the Judiciary Committee proceedings.
On the right of the chamber at the front sits the President's defence counsel - seven lawyers, led by Charles Ruff. Mr Ruff, White House chief counsel, summed up Mr Clinton's case in the House and will do the same in the Senate. Special tables have been made for each team.
The 100 senators, acting as jurors, have been given written guidelines for their comportment. They will sit on either side of the chamber: Republicans on the right, Democrats on the left. They must remain silent; whispering is ruled out, though no punishment has been set. They may ask questions and raise objections but only in writing and only through the presiding judge. They must ensure phones neither ring nor bleep.
They must rise "silently" when the Chief Justice arrives and leaves. When votes are cast, each senator must rise in turn and state their vote.
The first part of the trial is expected to last 10 days. Votes are then likely to be taken on whether to dismiss or to continue and whether witnesses should be called or more evidence be sought.