While Southern Illinois doesn’t have a universal definition, many Illinois residents consider Southern Illinois to be the bottom third of the State. It has an area code of (618) and the region contains over 1 million residents.
Wikipedia’s Southern Illinois page states that “Many Southern Illinois residents consider an area between Interstate 70 and Interstate 64 as the dividing line between the Central and Southern parts of the state.”
This area between Interstate 64 and Interstate 70 is shown to run east of St. Louis.
Some prominent Southern Illinois cities include Belleville, Carbondale, Collinsville, Edwardsville, Mt. Vernon, Marion, and Alton.
Southern Illinois has many different counties. A sample of these counties include Clinton, Madison, Monroe, St. Clair, Marion, Clay, Williamson, and Gallatin.