Before the silencing of Sen. Elizabeth Warren, D-Mass, few people outside the Senate had ever heard of Rule 19. -- the provision that Republican Majority Leader Mitch McConnell used to stop Warren from speaking on the Senate floor.

Did Elizabeth Warren break the rules? McConnell said she did. Some of this is a matter of interpretation, but if Warren did break the rules, her penalty is clearly an instance of selective enforcement. We’ll explain.

What actually happened on the Senate floor?

On Feb. 7, 2017, senators were debating whether to confirm President Donald Trump’s nominee for attorney general, Sen. Jeff Sessions, R-Ala.

On the floor, Warren was arguing against confirming Sessions. She quoted the late Sen. Edward Kennedy, D-Mass., who opposed Sessions confirmation as a federal judge in 1986. Kennedy at the time had called Sessions a "disgrace to the Justice Department."

Warren then read a letter, also from 1986, by Coretta Scott King, the widow of slain civil rights leader Martin Luther King, Jr. Her letter called out Sessions’ fitness for the office. In King’s words, Sessions "has used the awesome power of his office to chill the free exercise of the vote by black citizens in the district he now seeks to serve as a federal judge." (Sessions never won confirmation for the judgeship, but he was later elected to the Senate.)

As Warren was reading King’s letter, McConnell of Kentucky cited Rule 19, which states that "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." (It’s sometimes styled as Rule XIX.)

Senators proceeded to vote on whether Warren had indeed violated Rule 19, and they voted along party lines, 49-43, to silence Warren. That meant that she was effectively silenced for the rest of the debate.

"Sen. Warren was giving a lengthy speech," McConnell told his fellow lawmakers. "She had appeared to violate the rule. She was warned. She was given an explanation. Nevertheless, she persisted."

The episode sparked social media outrage among Warren’s supporters, and Warren proceeded to read King’s letter live from outside the Senate chamber. On Twitter, the hashtag #shepersisted alongside photos of people like Rosa Parks became ubiquitous.

Where did Rule 19 come from?

The Senate has long styled itself as a forum for civilized debate, dating back at least to Thomas Jefferson’s "A Manual of Parliamentary Practice for the Use of the Senate of the United States."

Jefferson’s 1801 manual included the provision that "no one is to disturb another in his speech by hissing, coughing, spitting, speaking or whispering to another, nor to stand up or interrupt him; nor to pass between the Speaker and the speaking member; nor to go across the chamber, or to walk up and down it, or to take books or papers from the (clerk’s) table, or write there."

Rule 19 wasn’t formalized, however, until 1902, when "John McLaurin, South Carolina's junior senator, raced into the Senate chamber and pronounced that state's senior senator, Ben Tillman, guilty of ‘a willful, malicious, and deliberate lie,’ " according to the Senate historian’s office. "Standing nearby, Tillman spun around and punched McLaurin squarely in the jaw. The chamber exploded in pandemonium as members struggled to separate both members of the South Carolina delegation. In a long moment, it was over, but not without stinging bruises both to bystanders and to the Senate's sense of decorum."

Too bad C-SPAN wasn’t around.

On Feb. 28, 1902, the Senate censured both members and added the provision of Rule 19 that, more than a century later, was used against Warren.

How frequently is Rule 19 invoked?

The Senate historian’s office said they don’t track the number of times particular rules are invoked. But "rarely," was the universal reply we heard from scholars.

Gregory Koger, a University of Miami political scientist, searched a data set he maiintains listing every Senate roll call vote from 1789 to 2014. The database doesn't specifically label invocations of Rule 19, but one of the codes used to classify votes are for allowing a lawmaker to speak after he or she has been reprimanded, which was a vote held regarding Warren on Feb. 7. Koger found only two previous votes on this question in the history of the Senate -- on Jan.. 29, 1915, and April 21, 1952.

"Senators frequently comment on other senators’ motivations and behavior in a way that some senators might see as imputing unworthy or unbecoming behavior to them," said Steven S. Smith, a political scientist at Washington University in St. Louis. "Most of the time, they let these comments pass. After all, to make an issue of it would draw even more attention to their opponents’ charges."

Part of the reason it has been invoked so infrequently is that there is a great deal of discretion in applying the rule. On one hand, any senator can invoke Rule 19. On the other hand, no senator is required to do so, and often, senators have preferred to let questionable remarks slide rather than making a battle out of it.

In addition, Smith said, "the definitions of ‘unworthy’ and ‘unbecoming’ are not provided. It is left to the presiding officer, subject to appeal, to decide the meaning on a case-by-case basis. An appeal leaves the matter to a majority of the Senate, as happened in Warren’s case."

The House of Representatives has a similar mechanism, referred to as a member’s words being "taken down." This has been used much more frequently than the Senate’s Rule 19 -- often at least once during a two-year House cycle, and on many occasions more than that, according to a 2011 Annenberg Public Policy Center study.

What are some examples where Rule 19 could have been invoked but wasn’t?

Koger suggests the bitter confirmation battle of former Sen. John Tower, R-Texas, for Defense Secretary. "Tower's personal failings were discussed as politely as possible, but senators were not Rule 19’ed," he said.

More recently, Sen. Ted Cruz, R-Texas, called out McConnell for allegedly lying to him about consideration of a provision in a transportation bill. On July 24, 2015, Cruz said, "I cannot believe he would tell a flat-out lie. ... What we just saw today was an absolute demonstration that not only what he told every Republican senator, but what he told the press over and over and over again, was a simple lie." No senator invoked Rule 19. (We took a closer look at Cruz’s comment here.)

Then, as recently as Feb. 2, Sen. David Perdue, R-Ga., slammed Senate Minority Leader Charles Schumer, D-N.Y., for shedding tears as he spoke out against President Donald Trump’s executive order freezing travel from several Muslim-majority countries.

"The minority leader’s ‘tear-jerking’ performance over the past weekend belonged at the Screen Actors Guild Awards, not in a serious discussion of what it takes to keep America safe," Purdue said. This didn’t draw any calls for Rule 19 sanctions, either.

Both the Cruz and Perdue examples "absolutely would have qualified" under any reading of Rule 19, Koger said. The fact that senators didn't pursue a Rule 19 challenge are "bright, shining examples of how senators tend to let things go."

So why did McConnell pursue Rule 19 in this instance?

His office said that every time a senator has been warned in recent years that they are pushing the limits, they have stopped what they’re doing. Warren, by contrast, pushed ahead despite the warning -- something that essentially has never happened recently, the office said.

And why were other Democratic senators -- all of them male, as Warren’s supporters noted -- able to read the King letter the next day without facing an invocation of Rule 19? Such comments would have been fair game, but no one who was on the floor at the time decided to call them on it. And Rule 19 cannot be invoked retroactively.

Does what Warren did represent an unusually low threshold for the use of Rule 19?

Scholars say it does, though it’s not entirely unprecedented.

Warren’s case set "an extremely low bar, because she was reading a public document that had already been recorded in the Congressional Record," Koger said.

The fact that Warren was reading a public document also caught Smith’s eye, though he cited a 1950 case in which the late Sen. Estes Kefauver, D-Tenn., was called out for "reading a newspaper article." Kefauver observed that, in reading the article, he was not "personally castigating" his colleague but instead was "reading the words of someone else."

The senators let the matter drop, but Smith said it could be considered a precedent for calling a senator to order for reading the words of others.

Would anyone get the same protections as Sessions did?

No. Rule 19 bars senators impugning other senators. Sessions may have been a cabinet nominee, but he was still a sitting senator. Another cabinet nominee who wasn’t a senator would not have received the protections Sessions did.