815 ILCS 205/9

Whenever, in any statute, act, deed, written or verbal, contract, or in any public or private instrument whatever, any certain rate of interest is or shall be mentioned, and no period of time is stated for which such rate is to be calculated, interest shall be calculated at the rate mentioned, by the year, in the same manner as if "per annum" or "by the year" had been added to the rate.