Ill. Const. art. IX, § 3

(a) A tax on or measured by income shall be at a non-graduated rate. At any one time there may be no more than one such tax imposed by the State for State purposes on individuals and one such tax so imposed on corporations. In any such tax imposed upon corporations the rate shall not exceed the rate imposed on individuals by more than a ratio of 8 to 5. (b) Laws imposing taxes on or measured by income may adopt by reference provisions of the laws and regulations of the United States, as they then exist or thereafter may be changed, for the purpose of arriving at the amount of income upon which the tax is imposed. (Source: Illinois Constitution.)