In economics, two goods whose demand tends to rise together because they are used together are called complementary goods.
Examples include cars and fuel, printers and ink, and left and right shoes. When the price of one complementary good falls, consumers may buy more of it and also increase purchases of the other. Economists measure this relationship with cross-price elasticity of demand, which is typically negative for complements.
Complementarity can be strong or weak. Products may be required together, commonly used together, or simply become more useful in combination. Complements can also be durable goods and ongoing services, such as a game console and compatible games. The relationship can change when technology, habits, or available alternatives change.
Complementary goods differ from substitutes, which can perform similar roles and often have positive cross-price elasticity. A change in the price of one good can affect demand for its complement, but the strength of the effect depends on how closely the products are linked and how large each cost is in the consumer’s budget.