(CNNMoney) - It all started with wine from Portugal. And it's served as the foundation to free trade.

A key trade theory turns 200 years old Wednesday. The marquee birthday comes at a time when President Trump is trying to redefine U.S. trade with other countries.

Published on April 19, 1817, British economist David Ricardo used a now famous example of wine from Portugal versus cloth from England to talk about trade in a paper entitled: "On the Principles of Political Economy and Taxation."

His theory was that it took fewer workers in Portugal, a country with a long history of wine making, to make wine than it does to make cloth in that country. And it took fewer workers to make cloth in England, a textile powerhouse at the time, than to produce wine there.

Thus, Portugal should export its wine to England, while England should export cloth to Portugal. Workers weren't the only factor. Other considerations were equipment, skills and the value of currencies.