<P> In economics, a complementary good or complement is a good with a negative cross elasticity of demand, in contrast to a substitute good . This means a good's demand is increased when the price of another good is decreased . Conversely, the demand for a good is decreased when the price of another good is increased . If goods A and B are complements, an increase in the price of A will result in a leftward movement along the demand curve of A and cause the demand curve for B to shift in; less of each good will be demanded . A decrease in price of A will result in a rightward movement along the demand curve of A and cause the demand curve B to shift outward; more of each good will be demanded . Basically this means that since the demand of one good is linked to the demand of another good, if a higher quantity is demanded of one good, a higher quantity will also be demanded of the other, and if a lower quantity is demanded of one good, a lower quantity will be demanded of the other . The prices of complementary goods are related in the same way: if the price of one good rises, so will the price of the other, and vice versa . With substitute goods, however, the price and quantity demanded of one good is related inversely to the price and quantity demanded of a substitute good, meaning that if the price or quantity demanded of one good rises, the price or quantity demanded of its substitute will fall . </P> <P> When two goods are complements, they experience joint demand . For example, the demand for razor blades may depend upon the number of razors in use; this is why razors have sometimes been sold as loss leaders, to increase demand for the associated blades . </P>

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