26 U.S.C. § 11: Tax imposed

A tax is hereby imposed for each taxable year on the taxable income of every corporation. The amount of the tax imposed by subsection (a) shall be 21 percent of taxable income. section 594 (relating to mutual savings banks conducting life insurance business), subchapter L (sec. 801 and following, relating to insurance companies), or subchapter M (sec. 851 and following, relating to regulated investment companies and real estate investment trusts). In the case of a foreign corporation, the taxes imposed by subsection (a) and section 55 shall apply only as provided by section 882. Except as otherwise provided in this subsection, the amendments made by subsections (a) and (b) [amending this section and sections 12, 280C, 453A, 527, 535, 594, 691, 801, 831, 832, 834, 852, 857, 860E, 882, 904, 1374, 1381, 1445, 1446, 1561, 6425, 6655, 7518, and 7874 of this title and repealing sections 1201 and 1551 of this title] shall apply to taxable years beginning after December 31, 2017 . The amendments made by subsection (b)(3) [amending sections 1445 and 1446 of this title] shall apply to distributions made after December 31, 2017 . The amendments made by subsection (b)(6) [amending section 1561 of this title ] shall apply to transfers made after December 31, 2017 .” The amendment made by subsection (a) [amending this section] shall apply to taxable years beginning on or after July 1, 1987 . “For treatment of taxable years which include July 1, 1987 , see section 15 of the Internal Revenue Code of 1986.” The amendments made by this section [amending this section and section 1561 of this title ] shall apply to taxable years beginning after December 31, 1983 . The amendments made by this subsection [probably should be “section”] shall not be treated as a change in a rate of tax for purposes of section 21 of the Internal Revenue Code of 1986 [formerly I.R.C. 1954].”