15 U.S.C. § 1665a

In responding orally to any inquiry about the cost of credit, a creditor, regardless of the method used to compute finance charges, shall state rates only in terms of the annual percentage rate, except that in the case of an open end credit plan, the periodic rate also may be stated and, in the case of an other than open end credit plan where a major component of the finance charge consists of interest computed at a simple annual rate, the simple annual rate also may be stated. The Bureau may, by regulation, modify the requirements of this section or provide an exception from this section for a transaction or class of transactions for which the creditor cannot determine in advance the applicable annual percentage rate.