Did NFL players who took a knee Sunday during the national anthem -- or sit out the anthem in their locker rooms -- break any rules, or even laws?

Social media posts on Sunday cited the NFL's rules and the U.S. Flag Code to attack or defend the protesting football players.

Let's start with NFL rules.

Many posts made the same claim: Teams and player were violating league rules and should be punished.

The tweets claimed they were highlighting language from the "NFL Rule Book":

"All players must be on the sideline for the National Anthem ... players on the field and bench area should stand at attention, face the flag ... Failure to be on the field by the start of the National Anthem may result in discipline."

The NFL Rule Book governs how the game is played, not what players do before the game.

The tweets' language appears to be from the NFL Game Operations Manual -- the league "bible" for everything that happens in a stadium on game day.

After former San Francisco 49er Colin Kaepernick's first protest in August 2016, the NFL backed off its policy that players "should stand at attention" and "face the flag."

"Players are encouraged but not required to stand during the playing of the national anthem," a league spokesman said at the time.

What about the three teams Sunday that failed to show up on the field for the anthem? NFL policy since 2009 says teams must be on the sideline, or face possible league discipline.

The three teams won't be punished. An NFL spokesman said: "There will be no discipline handed down."

There is also a federal statute -- the U.S. Flag Code -- governing how we should present the flag and conduct ourselves.

But there are no flag cops and penalties for ignoring the code.

Still, we've verified some violations here.

The NFL itself appears to be violating the flag code with those gigantic flags unfurled horizontally from sideline to sideline, and end zone to end zone.

Section 8 (c) "The flag should never be carried flat or horizontally, but always aloft and free."

What about that American flag tie worn proudly by Arizona congressman and Trump supporter Andy Biggs? Got to throw a flag on that:

Section 8(i) It should not be embroidered on such articles as cushions or handkerchiefs and the like.

A separate section of the federal code on the national anthem says this:

All present except those in uniform should stand at attention facing the flag with the right hand over the heart.

The language is the same as the NFL's: It uses the word "should," not the commands "shall" or "must."

Again, the code is advisory. "The Code functions simply as a guide to be voluntarily followed by civilians and civilian groups," according to a summary prepared for the U.S. Senate.

President Trump might be interested in Section 10 of the Flag Code.

With a simple proclamation, he could change any rule.