Cross-Elasticity of Demand

Equals the percentage change in quantity-demand for product A divided by the percentage change in the price of product B. If this is positive, product A is a substitute for product B, since an increase in the price of B causes an increase in the quantity-demanded of A. If negative, product A is complementary to product B rather than a substitute for it: an increase in the price of B causes a decrease in the quantity-demanded of product A.