Being Plastic is hard work. There are a lot of rules you have to follow if you want access to their lunch table.

Check out this incomplete list:

You can't wear a tank top two days in a row.
You can only wear your hair in a ponytail once a week.
Jeans or track pants may only be worn on Fridays.

Of course, they also wear pink on Wednesday.

These rules, and their de facto Wednesday uniform are symbols of The Plastics' unity. All three of them (four, once Cady joins the gang) donning pink every Wednesday says to their classmates, "We're together. All for one; one for all. Oh, and also, you can't sit with us."

To that end, wearing pink on Wednesdays also represents the fact that they're the only ones deemed worthy of being in the clique—kind of a "We're wearing pink, and you're not" deal. While it doesn't happen in the film, we're willing to bet that if Bethany or Jessica Lopez or—gasp!—Janis started wearing pink every Wednesday, too, Regina would blow a well-manicured gasket.

The Plastics' code of conduct is also a symbol of Regina's control over Gretchen, Karen, and, for a while, Cady. In reality, the rules are arbitrary, but, as we noted a moment ago, following them means being part of the group. Gretchen and Karen are more than willing to follow whatever dumb rule Regina makes up if it means they get to remain part of the social elite.

The rules' lack of actual meaning is reinforced when Regina wears sweatpants on a Monday, after her all-carb diet has caused her to pack on a few pounds, and Gretchen reminds her that sweatpants on a Monday are a no-no, and Regina therefore can't sit with him. "Whatever," Regina replies. "Those rules aren't real."

Nope, they're totally not, but until Cady and Janis started carrying out their revenge, those rules were super-effective at keeping Gretchen and Karen loyal to Regina, and, in that respect, they represent her Incredible Hulk-level power.