Federal income tax is imposed on citizens, residents, and U.S. corporations based on their worldwide income. To mitigate double taxation, a credit is allowed for foreign income taxes. This foreign tax credit is limited to that part of current year tax caused by foreign source income. Determining such part involves determining the source of income and allocating and apportioning deductions to that income. States tax resident individuals and corporations on their worldwide income, but few allow a credit for foreign taxes.