Dozens of NFL players knelt during the national anthem last Sunday, continuing Colin Kaepernick’s protest against racial injustice.

If you’re wondering if we might see a similar form of expression in the NBA, you might be surprised that there’s actually a rule against it.

Officially, the rule is this:

2) Players, coaches, and trainers are to stand and line up in a dignified posture along the sidelines or on the foul line during the playing of the National Anthem.

You can see it in the league’s official rulebook under the “Player/Team Conduct and Dress” section, below a rule requiring players to be “uniformly dressed” and above one requiring coaches to wear a sports coat.

The NBA did not reply to our request for comment.

However, on Friday, the NBA issued a memo to teams reminding them of the rule that requires players and coaches to stand for the national anthem.

Adam Silver confirmed to reporters that the league expects players to stand for the national anthem, citing the rule. He did not answer whether players would be penalized if the rule was broken.

Asked if a player will be penalized if he doesn't stand for the anthem, Adam Silver says such a situation would be dealt with if it arises. — Ian Begley (@IanBegley) September 28, 2017

NBA Players Association Executive Director Michele Roberts also suggested the players would address a kneeling situation with the league only should it happen.

“I haven’t discussed it with Adam because I haven’t needed to,” Roberts told Yahoo! Sports’ Chris Mannix. “I was not the executive director when the [Mahmoud Abdul-Rauf situation] happened, but I’m aware of the rule and the players are aware of the rule. We’ll see. This is a different commissioner. This is a different time.

“I disagree with the view that taking a knee is disrespectful,” Roberts continued. “But there is a rule. I don’t know that we’ll see because I don’t know if we’ll have a player do that. We’ve not yet had a player do that. But if it fact and if in fact the league responds by deciding to impose some penalty, then we will, as we’ve done for every player that wants to challenge the discipline, will support and defend that player.”

This weekend, we saw the NBA become embroiled in the national debate.

After Stephen Curry said he would not visit the White House to celebrate the team’s championship, President Donald Trump tweeted that he was disinvited. LeBron James then called Trump a “bum,” many NBA players expressed support for Curry, and the Warriors reacted in their own ways on Saturday.

On Saturday, Oakland Athletics rookie catcher Bruce Maxwell became the first MLB player to kneel during the anthem. (It’s worth repeating that he was not protesting the anthem itself.) We’ve seen protests from WNBA players and teams, most recently the L.A. Sparks before Game 1 of the WNBA Finals. We’ve seen protests from stars on the U.S. women’s national soccer team and at many collegiate and amateur sporting leagues.

But with the NBA season only a month away, there may be players who feel they should join the protest, too.

If any players do, it won’t be the first time the NBA’s national anthem rule has been tested.

The NBA had an anthem “controversy” two decades ago

Mahmoud Abdul-Rauf was a wiry 6’1 point guard with a lightning-quick jump shot. He would be the league’s all-time leading free-throw shooter by percentage, except he’s 39 attempts short of officially qualifying. During the 1995-96 season with the Denver Nuggets, he chose to sit or remain in the locker room during the national anthem.

No one noticed until a local reporter mentioned it in March. Once he did, many were outraged.

NBA commissioner David Stern suspended Abdul-Rauf for one game and negotiated a compromise: Abdul-Rauf would stand for the anthem, but he could do so with his eyes closed and head bowed in prayer. Abdul-Rauf was a devout Muslim and his religion was a major reason why he chose to originally begin his silent protest, so he complied with Stern’s request.

But Abdul-Rauf was traded to the Kings at the end of the season, lost sponsorships, and then lost his starting spot on his new team. When his contract expired in 1998, Abdul-Rauf was only 29, but no team showed interest in signing him. He was essentially blacklisted from the league, playing only 41 games three seasons later before officially retiring.

In 2017, things have changed.

The league has many outspoken advocates for social justice and has worked with players to help amplify those voices. In early September, Silver and Roberts co-signed a letter to players encouraging them to express their views on social issues affecting their communities.

But no one knows exactly how the NBA would respond if one of its players took his message to the court itself. We can’t know how the league will treat one of its players kneeling during the anthem until it happens.

Technically, kneeling during the anthem is not allowed by the NBA. We’ll see if the league enforces the rule should the situation arise, as it did two decades ago.