Every American knows that, if charged with a crime, he or she has a right to “a speedy and public trial by an impartial jury.”

What can a defendant do if the jury “decides” by the impartial flip of a coin?

Three centuries of common law cases suggest that the answer is “nothing.” That’s because courts usually won’t allow jurors to testify about what happened behind closed doors. Next week, the Supreme Court will hear a case testing whether that rule applies even when two jurors swear the deliberations contained overt racism against the defendant. A ruling either way would have important implications for a core part of the American criminal justice system––the right to an impartial trial by jury.

The rule that jurors cannot testify about what happened in the jury room comes from an enigmatic 1785 case called Vaise v. Delaval. In that case, two jurors provided sworn statements that the jury settled the case by playing “tossup”—that is, literally flipping a coin. In a 60-word opinion, Lord Mansfield, Chief Justice of the King’s Bench, refused to set aside the verdict.

He did not say that flipping a coin was proper. Indeed, he said, jurors who decided that way would be committing a “very high misdemeanor.” And he didn’t say that a jury’s conduct could never lead to a new trial.

What he did say—and what has become known as “the Mansfield rule”— was that “The Court cannot receive such an affidavit from any of the jurymen themselves.” Instead, “the Court must derive their knowledge from some other source: such as from some person having seen the transaction through a window, or by some such other means.”

In other words, as the current federal evidence rules phrase it, “[d]uring an inquiry into the validity of a verdict or indictment, a juror may not testify about any statement made or incident that occurred during the jury’s deliberations; the effect of anything on that juror’s or another juror’s vote; or any juror’s mental processes concerning the verdict or indictment.”

On the other hand, if people not on the jury learn of jury misconduct—by overhearing the deliberations, for example—a court may set a verdict aside. And jurors themselves can testify in three cases: if the jury relied on evidence from outside the record; if outsiders tampered with the jury; or if the jury foreperson wrote the wrong verdict on the form. They can also report misconduct to  the judge—if they do so before the verdict becomes final.

That brings us to the case of Miguel Angel Peña Rodriguez. In 2010, a Colorado jury convicted him of misdemeanor sexual contact and sexual harassment. The state charged that Peña Rodriguez had barged into the women’s room at a racetrack, turned out the light, and groped two underage girls before they could flee. Peña Rodriguez’s defense was that he wasn’t there. He had alibi testimony from a co-worker. The girls had misidentified him, he argued.

The jury agonized. After they told the judge they were split, the judge ordered them to try harder (lawyers call this instruction a “dynamite charge.”) Finally, the jury convicted Peña Rodriguez on the three misdemeanors, acquitting him on a felony count of sexual assault.

After the trial, two jurors told defense counsel that “Juror H.C.” had told the jury that, from his experience in law enforcement, the defendant must be guilty “because he’s Mexican and Mexican men take whatever they want.” They cited four other overtly racist statements, including one in which H.C. dismissed the alibi witness, also Latino, as an “illegal,” who must be lying. (Testimony showed he was a lawful immigrant.)

Peña Rodriguez’s lawyers sought a new trial; but the trial judge, and the Colorado Supreme Court, turned down the motion. The Mansfield rule is needed, the state court said, because jury verdicts should be final; jurors should be protected from potential harassment by defense counsel and from the fear of having to testify about their deliberations in open court; and courts need to maintain public confidence in the jury system.

Peña Rodriguez argued that these “policy” reasons cannot overcome the Sixth Amendment right to an impartial jury—particularly when something as odious as race prejudice is openly displayed in the jury room. (Lord Mansfield, an English judge, had no Sixth Amendment to contend with.) Racial bias may be more distasteful than other forms of jury prejudice, the state supreme court said, but the rule is the rule. To make any exception “would also require trial courts to make arbitrary judgments that hinge on the severity of a particular juror’s impropriety or the intensity of his bias. We decline to sanction such a haphazard process.”

Lower courts, state and federal, are split on this issue, with some—such as the Tenth Circuit and Pennsylvania—supporting the rule, and others, such as the First Circuit and Massachusetts, holding that race prejudice is so serious as to overcome it.

The Supreme Court’s own cases suggest the justices view racial bias at trial with distaste. In a 2016 case, Foster v. Chatman, the Court rebuked the state of Louisiana after a state prosecutor made overt efforts to keep African Americans off a jury. This Term, it will consider a case in which a jury pondering the death penalty heard racist evidence incompetently proffered by the defendant’s own lawyer.

On the other hand, the Court has repeatedly reaffirmed the Mansfield rule. In a 1987 case, it held, 5-4, that a court should not hear juror testimony that the other jurors were getting drunk at lunch, smoking pot, sniffing cocaine, selling each other drugs in the courthouse, and falling into drug-and-alcohol-induced stupors during afternoon testimony. “There is little doubt that post-verdict investigation into juror misconduct would in some instances lead to the invalidation of verdicts reached after irresponsible or improper juror behavior,” Justice Sandra Day O’Connor wrote for the majority. “It is not at all clear, however, that the jury system could survive such efforts to perfect it.”

Not only would juror testimony undermine the finality of a verdict, she wrote, but “full and frank discussion in the jury room, jurors’ willingness to return an unpopular verdict, and the community’s trust in a system that relies on the decisions of laypeople would all be undermined by a barrage of post-verdict scrutiny of juror conduct.” Instead, she said, parties must rely on other means of detecting juror bias, including questioning jurors before they are empaneled and observing them in the courtroom. And again, jurors witnessing misconduct can come forward before the verdict.

Two years ago, the Court unanimously reaffirmed the rule. In Warger v. Schauers, a juror in an auto-accident court swore that another juror had injected her own daughter’s auto accident—and fear of a lawsuit—into deliberations. At jury selection, the jurors had been asked whether there was any reason she could not be impartial, and had said no.

The no-impeachment rule, the Court said, does not violate the Sixth Amendment. But neither Supreme Court case concerned overt racism inside the jury room. The Warger court added a footnote: “There may be cases of juror bias so extreme that, almost by definition, the jury trial right has been abridged.”

If this case—with its clear evidence of shocking racism in the jury room—is not such a case, Peña Rodriguez might ask, what is?