There is a fourth type of trade protectionism that is not usually mentioned in text books, because it is subtle. That is a deliberated attempt by a country to lower its currency value, thereby making its exports cheaper and more competitive. However, this can ultimately result in retaliation, and start up a currency war. Countries can lower their currency's value through either a fixed exchange rate, like China's yuan, or by creating so much national debt that it has the same effect, like the U.S. dollar decline.