When we step onto an "L" car, we step into a social contract. As such, we're beholden to one simple rule: Don’t be a jerk.

Unfortunately, we Chicagoans aren’t all on the same page about which behaviors constitute jerkiness. Of course, there are the basics: Give up your seat for elderly and pregnant people, keep your music in your headphones, keep your bag off the seat next to you.

But trickier are the issues of nuance—faux pas that even seasoned CTA riders are guilty of. Ironically, some of these behaviors stem from our own insuppressible Midwestern politeness. Consider this a 200-level class in public transit etiquette.

The doorway lurker

Repeat after me: Move to the middle of the car. We Chicagoans have a nasty habit of stepping onto an "L" car and camping out in the train’s threshold for the duration of our ride. It's an honest mistake: It’s easier to exit a crowded train when you’re right at the doorway. Unfortunately, doing so forces folks boarding at subsequent stops to squeeze past you, brushing up against virtually everybody in the car to find some standing space at the center. So please: Sacrifice your door spot for the greater good of ridership and step allllll the way into the car. A little more. One more step. Good.

The premature detrainer

Commonly violated by suburbanites and tourists, this rule bears repeating for city-dwellers: Slow down! You have plenty of time to exit the train. Even in a crowded car, you don’t need to gather your belongings until the train pulls into your station. Getting up 90 seconds early takes up space and sparks an awkward shuffling of bodies with the train in motion. Trust that some of the people in front of you will detrain at your stop. And if they don't, a simple "coming out" will do the trick.

The open seat rejecter

Men of Chicago: We understand that chivalry has been drilled into your ego since birth, but you’ve got to undo this one. Too often, when a seat opens up in front of a man on the train, he'll refuse to sit down, presumably leaving the space for a woman. But on crowded cars, all this does is block anyone else from sitting in that open seat, and ultimately takes up more space. So gents, quash your pride. Take a seat for the team.

The suitcase wielder

This one’s tough. On one hand, our airports are accessible by train, and we should take advantage. On the other, your duffel bag is the size of a first grader and taking up three spaces, sir.

There’s no easy fix for commuting with a suitcase, but there are ways to ease the pain. First, sit at either end of the car, away from the doors. You can try sitting sideways in an aisle seat with your bag between your legs, or even tucking it beneath your seat. If all else fails, hold your bag in your lap. No luck? The least you could do is feel a bit more ashamed of all the space you're taking up.