55 ILCS 5/3-14019
The Commission shall, by its rules, provide for promotion in such classified service, on the basis of ascertained merit, examination and seniority in service, and shall provide, in all cases where it is practicable, that vacancies shall be filled by promotion. All examinations for promotion shall be competitive among such members of the next lower rank or grade as desire to submit themselves to such examination; provided, however, the commission, in order to create or increase competition, may combine ranks or grades for the purpose of promotion; and it shall be the duty of the commission to submit to the appointing power the names of not more than three applicants for each promotion having the highest rating. The method of examination and the rules governing the same and the method of certifying shall be the same as provided for applicants for original appointment.
55 ILCS 5/3-14019