<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> <P> Recent work in food consumption has elucidated the psychological processes by which the consumption of one good (e.g., cola) stimulates demand for its complements (e.g., a cheeseburger, pizza, etc .). Consumption of a food or beverage activates a goal to consume its complements: foods that consumers believe would produce super-additive utility (i.e., would taste better together). Eating peanut - butter covered crackers, for instance, increases the consumption of grape - jelly covered crackers more than eating plain crackers . Drinking cola increases consumers' willingness to pay for a voucher for a cheeseburger . This effect appears to be contingent on consumers' perceptions of what foods are complements rather than their sensory properties . </P> <P> An example of this would be the demand for cars and petrol . The supply and demand of cars is represented by the figure at the right with the initial demand D1 . Suppose that the initial price of cars is represented by P1 with a quantity demanded of Q1 . If the price of petrol were to decrease by some amount, this would result in a higher quantity of cars demanded . This higher quantity demanded would cause the demand curve to shift rightward to a new position D2 . Assuming a constant supply curve S of cars, the new increased quantity demanded will be at D2 with a new increased price P2 . Other examples include: Automobile and fuel, mobile phone and cellular service provider, Printer and Cartridge among others . </P>

If goods a and b are considered complements an increase in the price of a would cause