65 ILCS 20/21-15
If any ordinance of the city council be returned by the mayor to the council without his approval, the mayor may submit with the message stating his objections thereto, a substitute ordinance, and after the vote by which the original ordinance was passed is reconsidered, then, if no motion be made to pass such original ordinance, the veto of the mayor to the contrary notwithstanding, or if such motion be made and fails of adoption, such substitute ordinance may forthwith be considered, unless two members of the council demand the reference of such substitute ordinance to a committee and if such demand be made, such substitute ordinance shall be so referred unless two-thirds of the members of such council vote in favor of immediate consideration thereof, and if such ordinance receives the affirmative vote of a majority of all members of the council present and voting, shall take effect and be in force in lieu of such vetoed ordinance.
65 ILCS 20/21-15