Remember "complements complete each other." If you buy one, you are likely to buy the other. Think of peanut butter and jelly—the demand for one is tied to the other.
Complementary goods are products that are consumed together. The demand for one good is directly related to the demand for the other; if the price of one good decreases, the demand for its complement increases. Common examples include cars and gasoline, printers and ink cartridges, and coffee and sugar.