In economics, the cross elasticity of demand or cross-price elasticity of demand measures the responsiveness of the quantity demanded for a good to a change in the price of another good, ceteris paribus. It is measured as the percentage change in quantity demanded for the first good that occurs in response to a percentage change in price of the second good. For example, if, in response to a 10% increase in the price of fuel, the demand of new cars that are fuel inefficient decreased by 20%, the cross elasticity of demand would be: . A negative cross elasticity denotes two products that are complements, while a positive cross elasticity denotes two substitute products. Assume products A and B are complements, meaning that an increase in the demand for A accompanies an increase in the quantity demanded for B. Therefore, if the price of product B decreases, the demand curve for product A shifts to the right, increasing A's demand, resulting in a negative value for the cross elasticity of demand. The exact opposite reasoning holds for substitutes.