Jon Sopel
BBC North America Editor

It is hard to exaggerate the importance - and the self-importance - of the hundred men and women who make up the US Senate.

Flunkies accompany them wherever they go - a veritable army of interns take care of the tedious mundanities of life; they have legislative affairs directors, communications chiefs, fundraising teams, speechwriters.

But this week, and for only the third time in American history, the senators become humble - or not so humble - jurors.

The battle still to be had between Democrats and Republicans is whether witnesses will be called. One person I spoke to said you should think of this like the old cold war strategy of MAD - mutually assured destruction.

For every John Bolton called by the Democrats - Donald Trump's former national security adviser who was reportedly deeply unhappy about what was going on, there's a Hunter Biden - the former vice president's son - whom the Republicans might subpoena.

One impeachment expert told me he thought this would be a trial that would proceed with no witnesses at all.

Read Jon's full analysis here.

Anthony Zurcher
BBC North America reporter

Q: If Trump is acquitted, could he be charged in federal court after his term of office has ended? - Ethan, Birmingham

A: Nothing about the impeachment and removal process prevents a president from being tried for criminal violations after he or she leaves office. There are different standards in a criminal versus a Senate trial, of course. Impeachable offences need not be statutory crimes, and vice versa. But just because a president is forced from office does not shield him or her from legal exposure.

In fact, one of the most controversial episodes of Richard Nixon's Watergate scandal was when, after the 37th president had resigned rather than face impeachment, his successor, Gerald Ford, gave him a blanket pardon for all crimes he may have committed against the nation while in office. Otherwise, Nixon very probably would have faced indictment and trial.

The pardon, which Ford cited as necessary to heal the nation, was a highly unpopular move and contributed to his defeat two years later when he sought election to a full four-year presidential term.

Anthony answers more of your questions on the trial here.

The last president to be impeached was Bill Clinton, in 1998. The BBC's Nick Bryant covered Clinton's trial, he explains how it compares to Trump's.

Image source, Getty Images

In different circumstances, we might be referring to Donald Trump's impeachment as the fourth, not the third. Richard Nixon, the 37th president, resigned before facing certain impeachment.

Nixon's resignation came after his administration was linked to a break-in at the Democratic National Committee offices in 1972. The scandal, known as Watergate, unravelled through 1973 and evidence grew of a cover-up.

The house judiciary committee, including some Republican members, approved three articles of impeachment against him in 1974. After it was clear Nixon had lost his party’s support, he resigned rather than be impeached.

Image source, Drew Angerer/Getty Images

The seven Democratic House impeachment managers could begin making their case in the Senate as early as Wednesday. But first, senators will wrangle over a set of trial rules put forward yesterday by Republican leader Mitch McConnell.

In the proposed rules, McConnell has deferred debate over whether the Senate will call witnesses and new evidence until the middle of the trial.

The dispute over witnesses has become a key friction point between Democrats and Republicans in anticipation of the trial. McConnell has said he wants a speedy trial, but Democrats say they plan to force votes on calling additional witnesses like former national security adviser John Bolton.

“Sen. McConnell’s resolution is nothing short of a national disgrace," the Senate's top Democrat Chuck Schumer said on Monday.

There's quite a lot we don't know: whether Trump will appear; whether there'll be any witnesses; how long the trial will last. Here's what we do know:

• The trial will begin at 13:00 Washington time on Tuesday (18:00 GMT)
• It will run six days a week - Monday to Saturday
• Both sides will have 24 hours each - spread out over four days - to make their opening arguments
• Any new evidence, like witness testimony, would require a majority vote of 51 or more (there are 53 Republicans in the Senate, so more than the majority)
• Democrats have made no secret of their wish for witnesses and new documents, but with only 45 senators (plus two independents) they need at least four Republicans on their side for approval
• Republicans, including Senate majority leader Mitch McConnell, have made no secret of their wish for a fast-track acquittal

Image source, Reuters

For only the third time history, a US president is going on trial in the Senate.

It's all but certain that Donald Trump will be acquitted - his Republican party is in the majority in the Senate, and a two-thirds vote is required to convict him and remove him from office.

Before the trial begins in earnest, senators will need to agree the rules of the trial. Democrats would like to call witnesses and are not afraid of a longer process. The Republican Senate leader, Mitch McConnell has vowed to wrap it up as quickly as possible.

You can follow live coverage of the opening day here. Stay tuned.