"Cross-price elasticity of demand" is a measure of the responsiveness of the demand for one product to changes in the price of a different product. It is the ratio of percentage change in the former to the percentage change in the latter. If it is positive, the goods are called substitutes because a rise in the price of the other good causes consumers to substitute away from buying as much of the other good as before and into buying more of this good. If it is negative, the goods are called complements.