Columbia subject headings:
Duty-free importation--Law and legislation--Canada.
Duty-free importation--Law and legislation--United States.
Tariff--Law and legislation--Canada.
Tariff--Law and legislation--United States.
Harvard subject headings:
Princeton subject headings:
Free trade--Canada.
Free trade--United States.
Tariff--Law and legislation--Canada.
Tariff--Law and legislation--United States.