70 ILCS 1215/10
The Civil Service Board shall note of record the duties (whether imposed by law, official regulation or practice) of each office or place in the classified service. The Civil Service Board shall thereupon, by rule, fix lines for promotion from such several offices and places to superior offices or places in all cases where, in the judgment of the Board, the duties of such several positions directly tend to fit the incumbent for a superior position. In case of vacancy in superior offices or places which cannot be filled by reinstatement, the Civil Service Board shall hold promotional examinations to fill such vacancy within a reasonable time. Incumbents of offices or places next lower in the line so fixed shall be solely eligible for such examination, unless in the judgment of the Board, to be noted in its minutes with the grounds therefor, it is for the best interests of the service that original entrance examination for such vacancy be held. In promotional examinations, efficiency and seniority in service shall form a part of such examination, but combined shall not carry a total number of marks to exceed one-quarter (1/4) of the maximum marks obtainable in such examination. All examinations for promotion shall be competitive. The method of examination, the rules governing the same, and the method of certifying shall be the same as provided for in original entrance examination.
70 ILCS 1215/10