65 ILCS 5/11-12-9
If unincorporated territory is within one and one-half miles of the boundaries of two or more corporate authorities that have adopted official plans, the corporate authorities involved may agree upon a line which shall mark the boundaries of the jurisdiction of each of the corporate authorities who have adopted such agreement. On and after September 24, 1987, such agreement may provide that one or more of the municipalities shall not annex territory which lies within the jurisdiction of any other municipality, as established by such line. In the absence of such a boundary line agreement, nothing in this paragraph shall be construed as a limitation on the power of any municipality to annex territory. In arriving at an agreement for a jurisdictional boundary line, the corporate authorities concerned shall give consideration to the natural flow of storm water drainage, and, when practical, shall include all of any single tract having common ownership within the jurisdiction of one corporate authority. Such agreement shall not become effective until copies thereof, certified as to adoption by the municipal clerks of the respective municipalities, have been filed in the Recorder's Office and made available in the office of the municipal clerk of each agreeing municipality.
Any agreement for a jurisdictional boundary line shall be valid for such term of years as may be stated therein, but not to exceed 20 years, and if no term is stated, shall be valid for a term of 20 years. The term of such agreement may be extended, renewed or revised at the end of the initial or extended term thereof by further agreement of the municipalities.
In the absence of such agreement, the jurisdiction of any one of the corporate authorities shall extend to a median line equidistant from its boundary and the boundary of the other corporate authority nearest to the boundary of the first corporate authority at any given point on the line.
On and after January 1, 2006, no corporate authority may enter into an agreement pursuant to this Section unless, not less than 30 days and not more than 120 days prior to formal approval thereof by the corporate authority, it shall have first provided public notice of the proposed boundary agreement by both of the following:
The validity of a boundary agreement may not be legally challenged on the grounds that the notice as required by this Section was not properly given unless the challenge is initiated within 12 months after the formal approval of the boundary agreement.
An agreement that addresses jurisdictional boundary lines shall be entirely unenforceable for any party thereto that subsequently enters into another agreement that addresses jurisdictional boundary lines that is in conflict with any of the terms of the first agreement without the consent of all parties to the first agreement. For purposes of this Section, it shall not be considered a "conflict" when a municipality that is a party to a jurisdictional boundary line agreement cedes property within its own jurisdiction to another municipality not a party to the same jurisdictional boundary line agreement.
This amendatory Act of 1990 is declarative of the existing law and shall not be construed to modify or amend existing boundary line agreements, nor shall it be construed to create powers of a municipality not already in existence.
Except for those provisions to take effect prospectively, this amendatory Act of the 94th General Assembly is declarative of existing law and shall not be construed to modify or amend existing boundary line agreements entered into on or before the effective date of this amendatory Act, nor shall it be construed to create powers of a municipality not already in existence on the effective date of this amendatory Act.
65 ILCS 5/11-12-9