765 ILCS 225/3
For the purpose of the use of the Illinois Coordinate System, the State is divided into an "East Zone" and a "West Zone".
The area now included in the following counties constitutes the "East Zone": Boone, Champaign, Clark, Clay, Coles, Cook, Crawford, Cumberland, DeKalb, DeWitt, Douglas, DuPage, Edgar, Edwards, Effingham, Fayette, Ford, Franklin, Gallatin, Grundy, Hamilton, Hardin, Iroquois, Jasper, Jefferson, Johnson, Kane, Kankakee, Kendall, Lake, LaSalle, Lawrence, Livingston, McHenry, McLean, Macon, Marion, Massac, Moultrie, Piatt, Pope, Richland, Saline, Shelby, Vermilion, Wabash, Wayne, White, Will and Williamson.
The area now included in the following counties constitutes the "West Zone": Adams, Alexander, Bond, Brown, Bureau, Calhoun, Carroll, Cass, Christian, Clinton, Fulton, Greene, Hancock, Henderson, Henry, Jackson, Jersey, Jo Daviess , Knox, Lee, Logan, McDonough, Macoupin, Madison, Marshall, Mason, Menard, Mercer, Monroe, Montgomery, Morgan, Ogle, Peoria, Perry, Pike, Pulaski, Putnam, Randolph, Rock Island, St. Clair, Sangamon, Schuyler, Scott, Stark, Stephenson, Tazewell, Union, Warren, Washington, Whiteside, Winnebago and Woodford.
765 ILCS 225/3