735 ILCS 5/2-101
Except as otherwise provided in this Act, every action must be commenced (1) in the county of residence of any defendant who is joined in good faith and with probable cause for the purpose of obtaining a judgment against him or her and not solely for the purpose of fixing venue in that county, or (2) in the county in which the transaction or some part thereof occurred out of which the cause of action arose.
If a check, draft, money order, or other instrument for the payment of child support payable to or delivered to the State Disbursement Unit established under Section 10-26 of the Illinois Public Aid Code is returned by the bank or depository for any reason, venue for the enforcement of any criminal proceedings or civil cause of action for recovery and attorney fees shall be in the county where the principal office of the State Disbursement Unit is located.
If all defendants are nonresidents of the State, an action may be commenced in any county.
If the corporate limits of a city, village or town extend into more than one county, then the venue of an action or proceeding instituted by that municipality to enforce any fine, imprisonment, penalty or forfeiture for violation of any ordinance of that municipality, regardless of the county in which the violation was committed or occurred, may be in the appropriate court (i) in the county wherein the office of the clerk of the municipality is located or (ii) in any county in which at least 35% of the territory within the municipality's corporate limits is located.
735 ILCS 5/2-101