Sec 660 Jun 2026

Complex rules under Section 660 addressed situations where an overpayment on one tax period is later applied to an underpayment in another period – interest may not accrue during the gap if the IRS caused the delay.

Thus, when a modern tax lawyer or accountant references "Section 660," they are almost always referring to the interest computation regime codified primarily in and §6621 . However, because older court cases (pre-1986) cite Section 660 directly, understanding the original text is vital for precedent. sec 660

This compounding effect is a direct legislative modification of the original simple-interest Section 660. Complex rules under Section 660 addressed situations where

. However, "Section 660" also appears in several legal and regulatory contexts. SANS Institute This compounding effect is a direct legislative modification

Notice that SEC 660 is a flat percentage of the tax. It is interest —and interest never stops accruing until the balance is zero. If the underlying tax debt is $50,000, SEC 660 could add $10,000–$20,000 in pure time-cost, on top of the fraud or accuracy penalties.

In current practice, much of the operational detail of Section 660 has been superseded or expanded by and Section 6621 . However, understanding Section 660 in its original and amended form is crucial for tax professionals dealing with older tax years, audits extending back decades, or computational rules for interest accrual.

This means that even if the CSED expires on the principal tax, the SEC 660 interest may still be collectible because it accrued earlier. This is a complex area requiring a tax resolution specialist.