55 ILCS 5/4-1001
For the purpose of fixing the fees and compensation of county and township officers in this State, the several counties therein are hereby divided into 3 classes, according to population as ascertained by the most recent Federal Census, which classes shall be known as the first, second and third, as follows:
Counties containing a population of not exceeding 25,000 inhabitants shall be known as counties of the first class.
Counties containing a population over 25,000 and not exceeding 1,000,000 shall be known as counties of the second class.
Counties containing a population exceeding 1,000,000 shall be known as counties of the third class.
The fees and compensation of the several officers hereinafter named, shall be as provided by law in the respective classes of the counties to which they belong.
55 ILCS 5/4-1001