By Noah Feldman

The jockeying has already begun over the structure of President Donald Trump’s Senate trial.

Senate Majority Leader Mitch McConnell, R-Ky., has discussed it with the White House counsel; Senate Minority Leader Chuck Schumer, D-N.Y., has sent McConnell a letter proposing detailed protocols. All this action occurred even before the House of Representatives formally impeached Trump, and it might be making you wonder: Isn’t there some pre-existing trial protocol required by the Constitution? Do we really have to have a debate about how the trial is going to run before it actually happens?

The short answers are no, there isn’t a clear constitutional mandate for what the Senate trial should look like; and yes, there really does have to be a fight about what procedures the Senate will use in trying Trump. This seems like a crazy way to do things, but it reflects the framers’ recognition that impeachment as they knew it from England had always had a political side, and their reticence about putting too much detail in the Constitution.

The Constitution in article I, Section 3 gives the Senate “the sole Power to try all impeachments.” Then it provides just three brief rules about the trial itself.

The first rule is that “when sitting” for an impeachment trial, the senators “shall be on oath or affirmation.” The requirement that the Senators take an oath is the reason that we sometimes refer to the Senators as “jurors.” A juror is, technically, someone who swears an oath (from the Latin iurare, meaning to swear).

The Senators under oath get to vote on whether the president will be removed or not; but that’s pretty much where their resemblance to jurors in a court case ends. The senators can’t, for example, be restricted from hearing news about the subject of impeachment before or during the case.

The oath that the Senate uses (written, naturally, by the Senate) states that the senators “will do impartial justice according to the Constitution and laws.” That sounds jury-like. But the meaning of impartiality can’t be exactly the same as it would be for an ordinary jury member or judge, given that the senators are responsible to constituents and belong to political parties.

The second rule is: “When the president of the United States is tried, the chief justice shall preside.” The Constitution doesn’t, however, say exactly what powers the chief justice has while presiding. That’s left up to the Senate. And in its wisdom, or maybe its self-interest, the Senate has usually decided that any decision rendered by the chief justice could be overturned by a majority of, you guessed it, the Senate.

The third rule is that “no person shall be convicted without the concurrence of two thirds of the members present.” This rule makes the Senate trial different from a criminal trial, which requires unanimity for either guilt or innocence, and also from a civil trial, which requires either unanimity or a majority of jurors, depending on the state.

The key point here is that everything beyond these three simple rules is left to the Senate, which, like the House, has the constitutional authority to make its own rules of procedure.

If you’re designing a trial on a blank slate, the major issues are which witnesses can be called and which evidence can be considered. Pretty much both of these are up for grabs. Neither is specified in the impeachment rules that the Senate has in place.

By tradition drawn from England, members of the House of Representatives function as prosecutors in the Senate trial, and are called the impeachment managers or the House managers. Almost by definition, the managers are members of Congress who voted to impeach.

As for which evidence is allowed, there’s a good bit of variation in historical practice. Evidence that would not otherwise be admissible in court, like some hearsay, was allowed in the Clinton trial and in other modern impeachment trials in the Senate. But in the 1831 impeachment trial of Judge James Peck, the Senate debated the issue and decided that the trial should follow the same strict rules of procedure and evidence that would apply in an ordinary court.

The upshot is that a majority of the Senate can decide which witnesses will be heard and which evidence it will consider. It’s a funny way of conducting a trial. But an impeachment trial isn’t business as usual and probably could never be.

Feldman is a Bloomberg Opinion columnist. He is a law professor at Harvard University.