[2] The district covers parts of Madison and St. Clair counties, and all of Alexander, Franklin, Hamilton, Jackson, Jefferson, Monroe, Perry, Pulaski, Randolph, Union and Williamson counties, as of the 2011 redistricting which followed the 2010 census.

All or parts of Belleville, Cahokia, Carbondale, Collinsville, East St. Louis, Granite City, Herrin, Marion, Mt.

It will take in Monroe, Randolph, Clinton, Perry, Jackson, Union, Alexander, Pulaski, Massac, Johnson, Williamson, Jefferson, Marion, Clay, Effingham, Wayne, Hamilton, Saline, Pope, Hardin, Gallatin, White, Edwards, Wabash, Richland, Lawrence, Jasper, Crawford, Cumberland, and Clark Counties, most of St. Clair County, and half of Coles County.

They are partitioned by a Conrail line, Tanglewood Parkway, Donner Ridge, Hollywood Heights Rd, Oliver St, CSX Transportation Line, S Oak St, W 5th St, S Lincoln Ave, E US Highway 50, County Rd 218, Old O'Fallon Rd, Frank Scott Parkway E, N Green Mount Rd, S Green Mount Rd, Park Rd, S 59th St, Old St. Louis Rd, Illinois Highway 15, Rolling Acres Ln, Excellence Dr, Powdermill Creek, Cemetery Rd, and Illinois Highway 50.

The 12th district takes in the municipalities of Mascoutah, Lebanon, New Athens, Marissa, Millstadt, Smithton, Freeburg, Summerfield, Scott AFB, Rentchler, Floraville, Paderborn, Fayetteville, St. Libory, Darmstadt, and Lenzburg; most of Shiloh; half of O'Fallon; and part of Caseyville, Fairview Heights, and Belleville.