Complements are goods that are consumed together. Substitutes are goods where you can consume one in place of the other. The prices of complementary or substitute goods also shift the demand curve. When the price of a good that complements a good decreases, then the quantity demanded of one increases and the demand for the other increases. When the price of a substitute good decreases, the quantity demanded for that good increases, but the demand for the good that it is being substituted for decreases. Take a deeper dive into how changes in the prices of complements and substitutes affect the demand curve in this video. [5:58]