Sen. Elizabeth Warren, (D-Mass.), received a rare reproach Tuesday during debate over the confirmation of Sen. Jeff Sessions, (R-Ala.), as U.S. attorney general.

Warren was silenced under Senate debate rules as she was quoting from a letter written in 1986 by Coretta Scott King, widow of Martin Luther King Jr. The letter accused Sessions of using his power to “chill the free exercise of the vote by black citizens ... .”

According to Republican leadership aides, Warren was warned before she read the letter from King that she was close to violating the Senate’s rule against “impugning” a fellow senator.

An aide to Senate Majority Leader Mitch McConnell said the first warning on Tuesday night came after Warren read a statement written in 1986 by Sen Ted Kennedy in which he opposed Sessions' nomination as a federal judge. When Warren continued her speech and began to read from King's letter, McConnell, (R-Kty.) objected from the Senate floor.

McConnell took the action against Warren, his aide said, for the totality of her comments about Sessions, not just for reading King’s letter.

"She has been warned multiple times (not just Tuesday)," McConnell spokesman Don Stewart said. "And after additional warning today, she was found in violation of the rule. She appealed the ruling and lost."

What is Rule 19? Here’s a look at the rules that guide debate in the Senate.

What does the rule say?

Rule 19 covers how senators are to act during a debate in the Senate chamber. It’s made up of eight subsections. The one Warren is accused of violating is section 2. It reads, “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.”

What happens when someone disparages a fellow senator while speaking on the Senate floor?

Here is what Rule 19 requires if a senator is “called to order” by the presiding officer in the chamber over something negative said about a fellow senator during a debate:

“If any Senator, in speaking or otherwise, in the opinion of the Presiding Officer transgress the rules of the Senate the Presiding Officer shall, either on his own motion or at the request of any other Senator, call him to order; and when a Senator shall be called to order he shall take his seat, and may not proceed without leave of the Senate, which, if granted, shall be upon motion that he be allowed to proceed in order, which motion shall be determined without debate. Any Senator directed by the Presiding Officer to take his seat, and any Senator requesting the Presiding Officer to require a Senator to take his seat, may appeal from the ruling of the Chair, which appeal shall be open to debate.”

In other words, when Warren read the letter from King, after she was warned by McConnell about the statement she had read from Kennedy concerning Sessions, she was told to sit down. Then, by a vote of the Senate on McConnell’s objection to her language(50-43), she was silenced on the Senate floor until the debate over Sessions’ nomination is complete.

Does it happen often?

No, it does not happen often. Most senators refrain from disparaging fellow senators when speaking on the Senate floor, or if they are warned, they stop.

Here is the text of Rule 19 of the Rules of the Senate

Debate

1. (a) When a Senator desires to speak, he shall rise and address the Presiding Officer, and shall not proceed until he is recognized, and the Presiding Officer shall recognize the Senator who shall first address him. No Senator shall interrupt another Senator in debate without his consent, and to obtain such consent he shall first address the Presiding Officer, and no Senator shall speak more than twice upon any one question in debate on the same legislative day without leave of the Senate, which shall be determined without debate.

(b) At the conclusion of the morning hour at the beginning of a new legislative day or after the unfinished business or any pending business has first been laid before the Senate on any calendar day, and until after the duration of three hours of actual session after such business is laid down except as determined to the contrary by unanimous consent or on motion without debate, all debate shall be germane and confined to the specific question then pending before the Senate.

2. 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.

3. No Senator in debate shall refer offensively to any State of the Union.

4. If any Senator, in speaking or otherwise, in the opinion of the Presiding Officer transgress the rules of the Senate the Presiding Officer shall, either on his own motion or at the request of any other Senator, call him to order; and when a Senator shall be called to order he shall take his seat, and may not proceed without leave of the Senate, which, if granted, shall be upon motion that he be allowed to proceed in order, which motion shall be determined without debate. Any Senator directed by the Presiding Officer to take his seat, and any Senator requesting the Presiding Officer to require a Senator to take his seat, may appeal from the ruling of the Chair, which appeal shall be open to debate.

5. If a Senator be called to order for words spoken in debate, upon the demand of the Senator or of any other Senator, the exceptionable words shall be taken down in writing, and read at the table for the information of the Senate.

6. Whenever confusion arises in the Chamber or the galleries, or demonstrations of approval or disapproval are indulged in by the occupants of the galleries, it shall be the duty of the Chair to enforce order on his own initiative and without any point of order being made by a Senator.

7. No Senator shall introduce to or bring to the attention of the Senate during its sessions any occupant in the galleries of the Senate. No motion to suspend this rule shall be in order, nor may the Presiding Officer entertain any request to suspend it by unanimous consent.

8. Former Presidents of the United States shall be entitled to address the Senate upon appropriate notice to the Presiding Officer who shall thereupon make the necessary arrangements.