I live in Erie County in PA. I can borrow a book from any library that is either a) a part of the Erie County Library system or b) a library that has put their catalog listing into the Erie County Library catalog system. Frex, I live in Girard, but Girard's library is not a County library. However, their catalog is in the County's system, so its one card.
Also, there's a system called AccessPA, which is basically a PA-wide inter-library loan system. If my libraries don't have a book, I can request to borrow from a library in PA that does have it.