Twitter has a list. They're checking it twice.

Well, no, actually, it's much more complex than that. For the past two years, Twitter has been working with its Trust & Safety Council — a body of experts in online safety — to balance running a platform for free speech with curbing violence and harassment.

On Monday, Twitter put into effect new rules that are, in part, a result of many of those conversations. The rules are a major shift in the company's policies, which for years erred on the side of letting users post all sorts of vile content. Now, Twitter will take into account user behavior "on and off the platform" as it relates to affiliation with violent organizations.

In other words, context now matters on Twitter.

"This includes celebrating any violent act in a manner that may inspire others to replicate it or any violence where people were targeted because of their membership in a protected group. We will require offending Tweets to be removed and repeated violations will result in permanent suspension," a blog post on the new policies reads.

While Twitter's community standards previously banned violent extremism, monitoring "off platform" behavior and choosing to permanently ban users is a big departure from its old rules. The new rule helps close a loophole where someone affiliated with a hate group could still use Twitter by sharing a sanitized version of their message.

Twitter's embrace of free speech started early. CEO Jack Dorsey and Twitter's team of cofounders started Twitter in 2006 as place to broadcast thoughts publicly, no matter what they were. That openness famously led to Twitter serving as a platform for communication and activism during the Arab Spring.

It also opened the floodgates for racism and hate speech. ISIS most notably used Twitter as a place to spread propaganda. Dorsey has argued, in his defense of President Donald Trump’s use of the platform, that it’s better to have conversations in the open rather behind closed doors. But with these new rules, that's not the case for everyone anymore.

The new rules don't come out of nowhere. Recently, Twitter has grown more comfortable with permanently banning some high-profile accounts, such as Tim "Treadstone" Gionet, more widely known as Baked Alaska, conservative pundit Milo Yiannopoulos, and "pharma bro" Martin Shkreli. Meanwhile, Twitter verified Charlottesville "Unite the Right" rally organizer Jason Kessler, and then pulled the checkmark and its verification program after backlash.

Going forward with these new rules, Twitter admitted in Monday's blog post that the company will make mistakes.

"In our efforts to be more aggressive here, we may make some mistakes and are working on a robust appeals process. We’ll evaluate and iterate on these changes in the coming days and weeks, and will keep you posted on progress along the way," the post reads.

The change is a "slippery slope," said Susan Benesch, a member of Twitter's Trust & Safety Council. She serves as executive director of the Dangerous Speech Project and is a faculty associate at Harvard University's Berkman Klein Center for Internet & Society.

"That's tantamount to prohibiting certain opinions."

"If Twitter starts banning people who are affiliated with organizations that use or promote violence against civilians to further a cause, that's tantamount to prohibiting certain opinions: a very new policy for a platform that used to be known as the 'free speech wing of the free speech party,'" Benesch wrote in an email.

"It's not surprising, of course. If you're going to ban any kind of opinion, this is a logical place to start. Twitter staff may find themselves struggling to distinguish among guerrillas, terrorists, revolutionaries, and dissidents," she continued.

Critics of Twitter have argued the site no longer embraces free speech and is censoring its users with these new policies. Andrew Torba, former ad tech founder and now head of "free speech" social network Gab, has been calling for those critics to come over to his site.

Ahead of Monday's new rules, some people who believed they were at risk tweeted their accounts on Gab:

For those who think they might be gone from Twitter on 18th, just joined Gab, please follow me at: https://t.co/EBq09nREK0 — Kevin Xu (@kxu65) December 18, 2017

December 18, tomorrow, is the day Twitter has said it will make a clean sweep of everything"offensive." Twitter has already removed my blue check mark, so it may kill me off entirely tomorrow. I will be posting at Gab.ai under the same handle: @jartaylor pic.twitter.com/Ps0d4v9N88 — Jared Taylor (@jartaylor) December 17, 2017

If Twitter decides my conservative opinions too offensive for their site, you can find me at Gab.ai at https://t.co/FEVlRdPIL0. https://t.co/0LBZi4zkpE — Kim Priestap (@kimpriestap) December 17, 2017

Twitter declined to comment on which accounts will be removed, a.k.a. who's on its list and in what order. A Twitter spokesperson said they will review both first-person and bystander reports. The company also works with experts, such as members of their Trust & Safety Council, to identify what organizations and accounts should be permanently suspended.

The spokesperson said Twitter will not allow groups that:

Identify through their stated purpose, publications, or actions as an extremist group

Have engaged in, or currently engage in, violence (and/or the promotion of violence) as a means to further their cause

Target civilians in their acts (and/or promotion) of violence

The policy does not apply to military, government entities, or elected officials. Twitter also said it will consider making exceptions for groups that are engaged or have engaged in a "peaceful resolution," according to the spokesperson.

The new rules also ban hateful imagery or symbols in profile images or profile headers, as well as hate speech in usernames, display names, and profile bios. Twitter users can now report entire profiles for violating those rules, and Twitter will then require account owners to remove the violating media.

Hate speech is a broad term with many definitions and understandings, Benesch noted, which Twitter has attempted to address and to narrow.

"The one common thread is that it means to denigrate or attack a person or persons because of their membership in a group — usually ethnic, religious, social, etc," she wrote. "Twitter distinguishes between attacks on groups, such as #KillallMuslims, and attacks on specific people, usually aimed at those people with @ mentions."

For Twitter, examples of promoting violence include glorifying mass murders, terrorist attacks, and sexual assault, according to a spokesperson.

While Twitter users are right now kept in the dark of who is in and who is out, Twitter said it's trying to be more transparent when it chooses to take action. Twitter now labels profiles if someone is temporarily banned because they violated the media policy and were flagged for abusive content in their bio or handle.

For those users that believe they were wrongfully banned on Monday, Twitter has an online portal for appeals. Twitter also said it's still working to improve that process.