The collar counties are the five counties of Illinois that border on Chicago's Cook County. The collar counties (DuPage, Kane, Lake, McHenry, and Will) are tied to Chicago economically, but, like many suburban areas in the United States, have very different political leanings than does the core city. Chicago has long been a Democratic stronghold, and the collar counties are known for being historically Republican strongholds. However in recent years, they have become swing territory- voting Democratic in 2008, but switch back to Republican in 2010.

While the demographics of these suburban Chicago counties are fairly typical for American metropolitan areas, the term is apparently unique to this area. And because Chicago is so firmly entrenched in the Democratic column, and rural Downstate is so overwhelmingly Republican, the collar counties are routinely cited as being the key to any statewide election.

While the term is perhaps most often employed in political discussions, that is not its exclusive use.Barack Obama used the term in his speech before the Democratic National Convention in 2004.