Part I - General Authority

19 USC 1821 - Basic authority for trade agreements

(a) Determination by President; trade agreements; modification or continuance of existing duties 
Whenever the president determines that any existing duties or other import restrictions of any foreign country or the United States are unduly burdening and restricting the foreign trade of the United States and that any of the purposes stated in section 1801 of this title will be promoted thereby, the President may
(1) after June 30, 1962, and before July 1, 1967, enter into trade agreements with foreign countries or instrumentalities thereof; and
(2) proclaim such modification or continuance of any existing duty or other import restriction, such continuance of existing duty-free or excise treatment, or such additional import restrictions, as he determines to be required or appropriate to carry out any such trade agreement.
(b) Restrictions on decrease or increase in rate of duty 
Except as otherwise provided in this subchapter, no proclamation pursuant to subsection (a) of this section shall be made
(1) decreasing any rate of duty to a rate below 50 percent of the rate existing on July 1, 1962; or
(2) increasing any rate of duty to (or imposing) a rate more than 50 percent above the rate existing on July 1, 1934.

19 USC 1822 - Repealed. Pub. L. 93618, title VI, 602(d), Jan. 3, 1975, 88 Stat. 2072

Section, Pub. L. 87–794, title II, § 202, Oct. 11, 1962, 76 Stat. 872, made special provision for low-rate articles.

19 USC 1823 - Waiver of limitation on decrease in duty and negotiation and staging requirements for dicyandiamide and limestone

For purposes of this chapter, section 1821 (b)(1) of this title (relating to limit on decrease in duty), sections 1841, 1843, and 1844 of this title (relating to certain requirements concerning negotiations), and section 1883 of this title (relating to staging requirements) shall not apply with respect to dicyandiamide provided for in item 425.40 of the Tariff Schedules of the United States, and shall not apply with respect to limestone, when imported to be used in the manufacture of cement, provided for in item 513.34 of such Schedules.