When the dust settled, the Senate formally censured both men and passed a rule governing debate in the chamber that has stood to this day: “No Senator in debate shall, directly or indirectly, by any form of words impute to another Senator or to other Senators any conduct or motive unworthy or unbecoming a Senator.”

The fight that led to Rule XIX was the not the first time fisticuffs had broken out in the Senate, nor even its most famous brawl. That would be the time in 1856 when Representative Preston Brooks—also, it turns out, from South Carolina—attacked the antislavery Senator Charles Sumner of Massachusetts after his “Crime Against Kansas” speech. (It is now also against the rules to impugn not only a fellow senator, but to “refer offensively” to any individual state.) But the 1902 quarrel is the one that’s had the most lasting impact.

Jeff Sessions is President Trump’s nominee for attorney general, but until the Senate votes to confirm him to that post, he is still one of its 100 sitting members—he remains, in the politesse of the world’s greatest deliberative body, “the gentleman from Alabama.” And as such, he is afforded a greater protection from insult than any of the president’s other Cabinet nominees—even, in the case of Warren’s words on Tuesday night, from accusations contained in a 30-year-old letter involving his conduct before he became a senator.

What it means to violate Rule XIX is, of course, highly subjective, and its enforcement is extremely rare. It is not uncommon for senators to receive warnings or informal reprimands for remarks that their colleagues consider over the line. Often senators withdraw their contested comments from the record. But on Wednesday morning, the Senate historian’s office—which keeps extensive records on the chamber’s proceedings—was searching to discover if the Senate had ever enforced Rule XIX as it did on Tuesday night, by voting to silence Warren for the remainder of the current debate. Gregg Giroux of Bloomberg uncovered an example that came close in 1979, when then-Majority Leader Robert Byrd of West Virginia worked out a truce between Senators Lowell Weicker of Connecticut and John Heinz of Pennsylvania after Heinz invoked Rule XIX against Weicker for calling him an “idiot.”

McConnell said on Tuesday night that he sought a vote to silence Warren after she ignored warnings that reading from Coretta Scott King’s 1986 letter about Sessions violated Rule XIX. But as Democrats quickly pointed out, the majority leader took no action in 2015 when he was the victim of an even more egregious breach of the rule. Then it was a fellow Republican, Senator Ted Cruz of Texas, who called McConnell a liar on the Senate floor. The Kentucky senator, Cruz charged, had told “a simple lie” when he said there had been no deal struck to reauthorize the Export-Import Bank as part of a broader transportation bill.