Section 6662 imposes a 20% penalty on the portion of an underpayment attributable to any of several grounds, most commonly negligence or disregard of rules and regulations, and substantial understatement of income tax, which for most taxpayers means the understatement exceeds the greater of 10% of the correct tax or a specified dollar threshold. Certain more serious grounds, such as gross valuation misstatements, can trigger a higher 40% penalty rate instead.

The statute prevents penalty stacking: even where an underpayment could technically satisfy more than one Section 6662 ground, the taxpayer faces only a single accuracy-related penalty on that portion of the underpayment, not cumulative penalties for each applicable ground. A taxpayer can avoid the penalty by establishing reasonable cause and good faith, a fact-intensive defense frequently built around good-faith reliance on a qualified tax professional's advice, provided the reliance itself was reasonable given all the facts.

Because accuracy-related penalty exposure depends directly on whether the underlying position met the substantial authority standard and whether a reasonable-cause defense is available, Juricratic links the penalty-tier dial to the same authority-weighting dial used for the substantial authority entry, so a simulation of the position's underlying strength flows through consistently to the resulting penalty exposure estimate.

Counsel defending against a proposed accuracy-related penalty builds the record around whichever defense fits the facts — either that the position met the substantial authority standard for the understatement ground, or that the taxpayer had reasonable cause and acted in good faith, commonly through documented reliance on professional tax advice — since either defense, if established, can eliminate the 20% penalty regardless of whether the underlying position is ultimately sustained.