- 18 - filing of a tax return is not excused by the taxpayer’s reliance on an agent, and such reliance is not ‘reasonable cause’ for a late filing under § 6651(a)(1).” United States v. Boyle, 469 U.S. 241, 252 (1985). Respondent’s determination that petitioners are liable for the addition to tax for failing timely to file their 2001 Federal income tax return is sustained. D. Section 6662 Accuracy-Related Penalty The final issue for decision is whether petitioners are liable for an accuracy-related penalty under section 6662(a) for 2001. Section 6662(a) imposes a penalty equal to 20 percent of any underpayment of tax that is attributable to negligence or disregard of rules or regulations, or a substantial understatement of income tax. See sec. 6662(a) and (b)(1) and (2). An “understatement” of income tax is defined as the excess of the tax required to be shown on the return over the tax actually shown on the return. Sec. 6662(d)(2)(A). An understatement is “substantial” if it exceeds the greater of 10 percent of the tax required to be shown on the return, or $5,000. Sec. 6662(d)(1)(A). Whether the accuracy-related penalty is applied because of negligence or disregard of rules or regulations, or a substantial understatement of income tax, section 6664 provides a defense ifPage: Previous 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 NextLast modified: March 27, 2008