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.
Tariff
A crucial assumption in both the classical and neoclassical formulation of comparative advantage theory is that trade is balanced, which means that the value of imports is equal to the value of each country's exports. The volume of trade may change, but international trade will always be balanced at least after a certain adjustment period. The balance of trade is essential for theory because the resulting adjustment mechanism is responsible for transforming the comparative advantages of production costs into absolute price advantages. And this is necessary because it is the absolute price differences that determine the international flow of goods. Since consumers buy a good from the one who sells it cheapest, comparative advantages in terms of production costs must be transformed into absolute price advantages. In the case of floating exchange rates, it is the exchange rate adjustment mechanism that is responsible for this transformation of comparative advantages into absolute price advantages. In the case of fixed exchange rates, neoclassical theory suggests that trade is balanced by changes in wage rates.