26 U.S.C. § 3201

(a) Tier 1 tax In addition to other taxes, there is hereby imposed on the income of each employee a tax equal to the applicable percentage of the compensation received during any calendar year by such employee for services rendered by such employee. For purposes of the preceding sentence, the term “applicable percentage” means the percentage equal to the sum of the rates of tax in effect under subsections (a) and (b) of section 3101 for the calendar year. (b) Tier 2 tax In addition to other taxes, there is hereby imposed on the income of each employee a tax equal to the percentage determined under section 3241 for any calendar year of the compensation received during such calendar year by such employee for services rendered by such employee. (c) Cross reference For application of different contribution bases with respect to the taxes imposed by subsections (a) and (b), see section 3231(e)(2).