A good which is purchased alongside another good as they need to be consumed together to satisfy a want or desire.
Below is a diagram to illustrate complementary goods in a demand and supply framework. Pasta and pasta sauce are two goods which need to be consumed together and therefore are complementary goods. As the diagrams below show, an increase in supply of pasta leads to a fall in the price due to excess supply. If the price of pasta falls, demand increases for pasta but this will also cause the demand for pasta sauce to increase as they are complementary goods. Hence why the demand curve for pasta sauce shifts outwards.
