This study investigates the choice between complementary and parallel alliances in a market with vertical and horizontal externalities. One composite goods firm competes with two components producers, each providing a complementary component of a differentiated composite good. Although the joint profits from a parallel alliance between the composite goods firm and a components producer are always larger than those from a complementary alliance between components producers, through Nash bargaining, a components producer prefers the complementary (parallel) alliance when the degree of product differentiation is sufficiently large (small). Combined with the result that a complementary alliance is socially preferable, our findings provide meaningful implications for antitrust policy.