<P> In economics , a complementary goods or complement is a goods with a negative cross elasticity of demand , in contrast to a substitute goods . This means a good 's demand is increased when the price of another goods is decreased . Conversely , the demand for a goods is decreased when the price of another goods 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 goods 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 goods will be demanded . Basically this means that since the demand of one goods is linked to the demand of another goods , if a higher quantity is demanded of one goods , a higher quantity will also be demanded of the other , and if a lower quantity is demanded of one goods , 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 goods rises , so will the price of the other , and vice versa . With substitute goods , however , the price and quantity demanded of one goods is related inversely to the price and quantity demanded of a substitute goods , meaning that if the price or quantity demanded of one goods rises , the price or quantity demanded of its substitute will fall . </P>
if the price of crackers goes up when the price of cheese goes down crackers and cheese are
complements