Illinois Statutes Chapter 65. Municipalities §-25 5/3.1-20-25. Redistricting a city

(a) In the formation of wards, the number of inhabitants of the city immediately preceding the division of the city into wards shall be as nearly equal in population, and the wards shall be of as compact and contiguous territory, as practicable. Wards shall be created in a manner so that, as far as practicable, no precinct shall be divided between 2 or more wards.

(b) Whenever an official decennial census shows that a city contains more or fewer wards than it is entitled to, the city council of the city, by ordinance, shall redistrict the city into as many wards as the city is entitled. This redistricting shall be completed not less than 30 days before the first day set by the general election law  1 for the filing of candidate petitions for the next succeeding election for city officers. At this election there shall be elected the number of alderpersons to which the city is entitled, except as provided in subsection (c).

(c) If it appears from any official decennial census that it is necessary to redistrict under subsection (b) or for any other reason, the city council shall immediately proceed to redistrict the city and shall hold the next city election in accordance with the new redistricting. At this election the alderpersons whose terms of office are not expiring shall be considered alderpersons for the new wards respectively in which their residences are situated. At this election, in a municipality that is not a newly incorporated municipality, a candidate for alderperson may be elected from any ward that contains a part of the ward in which he or she resided at least one year next preceding the election that follows the redistricting, and, if elected, that person may be reelected from the new ward he or she represents if he or she resides in that ward for at least one year next preceding reelection. If there are 2 or more alderpersons with terms of office not expiring and residing in the same ward under the new redistricting, the alderperson who holds over for that ward shall be determined by lot in the presence of the city council, in the manner directed by the council, and all other alderpersons shall fill their unexpired terms as alderpersons-at-large. The alderpersons-at-large, if any, shall have the same powers and duties as all other alderpersons, but upon the expiration of their terms the offices of alderpersons-at-large shall be abolished.

(d) If the redistricting results in one or more wards in which no alderpersons reside whose terms of office have not expired, 2 alderpersons shall be elected in accordance with Section 3.1-20-35, unless the city elected only one alderperson per ward pursuant to a referendum under subsection (a) of Section 3.1-20-20.

(e) A redistricting ordinance that has decreased the number of wards of a city because of a decrease in population of the city shall not be effective if, not less than 60 days before the time fixed for the next succeeding general municipal election, an official census is officially published that shows that the city has regained a population that entitles it to the number of wards that it had just before the passage of the last redistricting ordinance.