Cover Free

How to get the passwords (for free cover) for Chicago nightclubs?

I friends always find a way to get those so call passwords. They are more like a word, or a letter sometime.
You say it at the door in the nightclub. and you can get in for free.
I was wondering how do people general get that?
Do they have to pay to get it?
Clubs:sound bar, vision, buzz, republic, …etc

You probably have to know someone in order to get in for free, or via these so-called ‘passwords.

John Mayer – Free Fallin’ (Live at the Nokia Theatre)