Tariff
A tariff is called an optimal tariff if it's set to maximize the welfare of the country imposing the tariff. It is a tariff derived by the intersection between the "trade indifference curve" of that country and the offer curve of another country. In this case, the welfare of the other country grows worse simultaneously, thus the policy is a kind of "beggar thy neighbor policy". If the offer curve of the other country is a line through the origin point, the original country is in the "condition of a small country", so any tariff worsens the welfare of the original country.
International economics
Economists’ findings about the benefits of trade have often been rejected by government policy-makers, who have frequently sought to protect domestic industries against foreign competition by erecting barriers, such as tariffs and import quotas, against imports. Average tariff levels of around 15 per cent in the late 19th century rose to about 30 percent in the 1930s, following the passage in the United States of the Smoot–Hawley Tariff Act. Mainly as the result of international agreements under the auspices of the General Agreement on Tariffs and Trade (GATT) and subsequently the World Trade Organization (WTO),
average tariff levels were progressively reduced to about 7 per cent during the second half of the 20th century, and some other trade restrictions were also removed. The restrictions that remain are nevertheless of major economic importance: among other estimates,
the World Bank estimated in 2004 that the removal of all trade restrictions would yield benefits of over $500 billion a year by 2015.