Comparative advantage refers to the ability of a person or a country to produce a particular good or service at a lower marginal and opportunity cost over another. Comparative advantage was first described by David Ricardo who explained it in his 1817 book "On the Principles of Political Economy and Taxation" in an example involving England and Portugal. In Portugal it is possible to produce both wine and cloth with less labour than it would take to produce the same quantities in England. However the relative costs of producing those two goods are different in the two countries. In England it is very hard to produce wine, and only moderately difficult to produce cloth. In Portugal both are easy to produce. Therefore, while it is cheaper to produce cloth in Portugal than England, it is cheaper still for Portugal to produce excess wine, and trade that for English cloth. Conversely England benefits from this trade because its cost for producing cloth has not changed but it can now get wine at a lower price, closer to the cost of cloth. The conclusion drawn is that each country can gain by specializing in the good where it has comparative advantage, and trading that good for the other.