Federal Rule of Evidence 403, described as the cornerstone of the entire Code, provides, “Although relevant, evidence may be excluded if its probative value is substantially outweighed by the danger of unfair prejudice, confusion of the issues, or misleading the jury, or by considerations of undue delay, waste of time, or needless presentation of cumulative evidence.” It is an iconic rule. The Rule, along with its state law counterparts, is the primary trial management tool in the American court system. FRE 403 is the means judges use to keep trials “on track” and “on task,” by excluding “unquestioned” relevant evidence from the jury in order to promote accuracy and efficiency. The Rule also has a major constitutional problem -- one that has not even been identified, much less resolved. The Sixth and Seventh Amendments to the Constitution -- respectively guaranteeing the right to a criminal jury and a civil jury -- make it a right of all Americans that juries, not judges, decide what weight to give to evidence. Thus, constitutionally, there is no way to keep relevant evidence from the jury, at least not without a countervailing constitutional value. There is no countervailing constitutional value. Efficiency is not a constitutional value. Accuracy is not a constitutional value. One might argue that a rule premised on trusting judges more than juries to weigh evidence makes for better justice, but such a rule is at odds with the Framers’ decision. 19th Century evidence theorists were the first to propose efficiency or accuracy as a basis to exclude relevant evidence. This was decades after the ratification of the Constitution, and the adoption Sixth and Seventh Amendments. Yet neither these theorists, nor any judge or writer since, has ever addressed the possible constitutional barrier to the Rule.The implications of this conclusion -- that the desirability of efficient trials cannot justify declining to honor the constitutional prerogative of juries -- are potentially profound. Large swaths of federal jurisprudence -- both criminal and civil -- are grounded in making trials more efficient and accurate. If efficiency and accuracy are insufficient to overcome the prerogative of the jury, then potentially lots of rules are built on illusory foundations.