40 ILCS 5/6-210
Service rendered by a fireman, as a regularly appointed and sworn policeman of the city shall be included, for the purposes of this Article, as if such service were rendered as a fireman of the city. Salary received by a fireman for any such service as a policeman shall be considered, for the purposes of this Article, as salary received as a fireman. Any annuity payable to a fireman under this Article shall be reduced by any pension or annuity payable to him from any policemen's annuity and benefit fund in operation in the city, and any member entering service after January 1, 2011 shall not be given service credit in this fund for any period of time in which the member is in receipt of retirement benefits from any annuity and benefit fund in operation in the city.
Any policeman who becomes a fireman, subsequent to July 1, 1935, may contribute to the fund an amount equal to the sum which would have accumulated to his credit from deductions from salary for annuity purposes if he had been contributing to the fund such sums as he contributed for annuity purposes to the policemen's annuity and benefit fund, and no credit for periods of service rendered by him in the police department shall be allowed, under this Article, except as to such periods for which he made contributions to the policemen's annuity and benefit fund, provided he has made the payments required by this Article.
40 ILCS 5/6-210