Introduction
Cross price elasticity of demand: When change in quantity demanded of one commodity is measured with respect to change in price of the other commodity, it is called cross elasticity of demand.
Substitute goods : Two goods are substitute if the products could be used for the same purpose by the consumers. That is, a consumer perceives both goods as similar or comparable, so that having more of one good causes the consumers to desire less of the other good.
Complementary good : A complementary good is a product or services that add value to another. In other words, they are two goods that the consumer uses together.
Which means that the cross-price elasticity of demand of substitute good is positive and the cross-price elasticity of demand of complementary good is negative.