19 USC 2111 - Basic authority for trade agreements

(a) Presidential authority to enter into agreement; 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 the purposes of this chapter will be promoted thereby, the President
(1) during the 5-year period beginning on January 3, 1975, may enter into trade agreements with foreign countries or instrumentalities thereof; and
(2) may proclaim such modification or continuance of any existing duty, such continuance of existing duty-free or excise treatment, or such additional duties, as he determines to be required or appropriate to carry out any such trade agreement.
(b) Limitation on authority to decrease duty 

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(1) Except as provided in paragraph (2), no proclamation pursuant to subsection (a)(2) of this section shall be made decreasing a rate of duty to a rate below 40 percent of the rate existing on January 1, 1975.
(2) Paragraph (1) shall not apply in the case of any article for which the rate of duty existing on January 1, 1975, is not more than 5 percent ad valorem.
(c) Limitation on authority to increase duty 
No proclamation shall be made pursuant to subsection (a)(2) of this section increasing any rate of duty to, or imposing a rate above, the higher of the following:
(1) the rate which is 50 percent above the rate set forth in rate column numbered 2 of the Tariff Schedules of the United States as in effect on January 1, 1975, or
(2) the rate which is 20 percent ad valorem above the rate existing on January 1, 1975.
[1] Not printed in the Federal Register. The text of the Geneva (1979) Protocol to the General Agreement in part 1 of Annex I has been printed by the Contracting Parties to the General Agreement on Tariffs and Trade in four volumes entitled Geneva (1979) Protocol to the General Agreements on Tariffs and Trade. The Agreement with the Hungarian People’s Republic in part 6 of Annex I has been printed in House Document 96–153, vol. 1, p. 703. The general provisions of all the agreements in parts 1 to 6 of annex I, but not schedules of concessions by other parties, will be printed in the Customs Bulletin. The texts of all these agreements will be printed in Treaties and Other International Acts Series, and in the bound volumes of United States Treaties and Other International Agreements.