Two goods are complements when a decrease in the price of one good increases the quantity demanded of the other good. This is because complementary goods are typically consumed together, so if one becomes cheaper and more attractive to buy, the demand for its complement also rises since both goods are used jointly (e.g., cereal and milk). Conversely, an increase in the price of one would decrease the quantity demanded of the other good. This relationship reflects that complementary goods add value to each other in consumption, so a price decrease in one positively affects the demand for the other.