In the Burdett and Judd (1983) model retailers are selling only one good and thus setting only one price. As a result, while lowering it to attract the shoppers they also lose sure prot they earn from the captives. In contrast, if retailers are selling two complements, they can keep the sum of the two prices constant at the joint reservation value of the two goods (thus ensuring that the prot earned from the captives is unchanged) and lower one of the prices, engaging aggressively in a price competition for the shoppers. Joint discrimination is impossible if the two goods are substitutes. Unlike complements, substitutes that a retailer.
