Quick search by citation:

26 U.S. Code § 6684 - Assessable penalties with respect to liability for tax under chapter 42

If any person becomes liable for tax under any section of chapter 42 (relating to private foundations and certain other tax-exempt organizations) by reason of any act or failure to act which is not due to reasonable cause and either—
such person has theretofore been liable for tax under such chapter, or
such act or failure to act is both willful and flagrant,
then such person shall be liable for a penalty equal to the amount of such tax.
Editorial Notes

1987—Pub. L. 100–203 inserted “and certain other tax-exempt organizations” after “private foundations” in parenthetical.

Statutory Notes and Related Subsidiaries
Effective Date of 1987 Amendment

Amendment by Pub. L. 100–203 applicable to taxable years beginning after Dec. 22, 1987, see section 10712(d) of Pub. L. 100–203, set out as an Effective Date note under section 4955 of this title.

Effective Date

Section effective Jan. 1, 1970, see section 101(k)(1) of Pub. L. 91–172, set out as a note under section 4940 of this title.