The Internal Revenue Code is today embodied as Title 26 of the United States Code (26 U.S.C.). The federal income tax law applies to all residents and citizens of the U.S. Corporations must also file income tax returns and are subject to special corporate filing rules. In addition to federal income taxes, most states subject its residents to income taxes.
Some of the terms used in income tax law are "Gross income", generally defined as "all income from whatever source derived;" "taxable income" and "adjusted gross income". Court cases are continually interpreting the definition of taxable income. Income tax is calculated on a formula which takes into consideration whether it is paid jointly by a married couple, the number of dependents of the taxpayers, special breaks for ages over 65, disabilities and other factors.