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.
Customs duties in the United States
Customs duty rates may be expressed as a percentage of value or dollars and cents per unit. Rates based on value vary from zero to 20% in the 2011 schedule. Rates may be based on relevant units for the particular type of goods (per ton, per kilogram, per square meter, etc.). Some duties are based in part on value and in part on quantity. The United States applies a Customs tariff that is among the lowest in the world: 3% on average.