Complementary goods are commodities that depend on each other such that in increase in the price of one good causes a decrease in the demand for the related good. For example, an increase in the price of french toast could cause a decrease in the demand for maple syrup, because those two goods are often consumed together. An increase in the price of salad dressing could cause a decrease in demand for salad.
Complementary goods are the opposite of [[substitute goods]].