26 U.S.C. § 6204: Supplemental assessments

(a) General ruleThe Secretary may, at any time within the period prescribed for assessment, make a supplemental assessment whenever it is ascertained that any assessment is imperfect or incomplete in any material respect. (b) Restrictions on assessmentFor restrictions on assessment of deficiencies in income, estate, gift, and certain excise taxes, see section 6213.