26 U.S.C. § 39: Carryback and carryforward of unused credits

a business credit carryback to the taxable year preceding the unused credit year, and a business credit carryforward to each of the 20 taxable years following the unused credit year, The entire amount of the unused credit for an unused credit year shall be carried to the earliest of the 21 taxable years to which (by reason of paragraph (1)) such credit may be carried. The amount of the unused credit for the unused credit year shall be carried to each of the other 20 taxable years to the extent that such unused credit may not be taken into account under section 38(a) for a prior taxable year because of the limitations of subsections (b) and (c). this section shall be applied separately from the business credit (other than the marginal oil and gas well production credit), paragraph (1) shall be applied by substituting “each of the 5 taxable years” for “the taxable year” in subparagraph (A) thereof, and by substituting “25 taxable years” for “21 taxable years” in subparagraph (A) thereof, and by substituting “24 taxable years” for “20 taxable years” in subparagraph (B) thereof. this section shall be applied separately from the business credit (other than the applicable credit), paragraph (1) shall be applied by substituting “each of the 3 taxable years” for “the taxable year” in subparagraph (A) thereof, and by substituting “23 taxable years” for “21 taxable years” in subparagraph (A) thereof, and by substituting “22 taxable years” for “20 taxable years” in subparagraph (B) thereof. the amounts determined under paragraphs (1) and (2) of section 38(a) for such taxable year, plus the amounts which (by reason of this section) are carried back to such taxable year and are attributable to taxable years preceding the unused credit year. The amount of the unused credit which may be taken into account under section 38(a)(1) for any succeeding taxable year shall not exceed the amount by which the limitation imposed by section 38(c) for such taxable year exceeds the sum of the amounts which, by reason of this section, are carried to such taxable year and are attributable to taxable years preceding the unused credit year. No portion of the unused business credit for any taxable year which is attributable to a credit specified in section 38(b) or any portion thereof may be carried back to any taxable year before the first taxable year for which such specified credit or such portion is allowable (without regard to subsection (a)).