### Question
How does Interest on Underpayments define 30-day letter?


### Answer
Under *26 U.S.C. § 6603*, **30-day letter** is defined as:

> the first letter of proposed deficiency which allows the taxpayer an opportunity for administrative review in the Internal Revenue Service Office of Appeals

#### Citation:
  - *26 U.S.C. § 6603*
  - Heading: **Subtitle F-Procedure and Administration > CHAPTER 67-INTEREST > Subchapter A-Interest on Underpayments**

