International trade theory recognizes three fundamental reasons for countries to trade: comparative advantage (to exploit differences in countries' tastes, technologies, or factor endowments), economies of scale (to concentrate on fewer tasks in order to produce more efficiently), and imperfect competition (to expose firms to more competition). Comparative advantage has always been dominant in trade theory, although economies of scale also long played a secondary role. In practice, scale economies occur in great variety, so a classification of the more important attributes is useful. Scale economies are internal if the individual firm can reduce average costs by operating at a higher scale. Economies of scale may depend on the scale of operations within a nation or on the scale of operations globally. Increasing returns maybe a property of manufacturing generally or of individual manufactured goods. Though scale economies were in trade theory from the beginning, their role was basically tangential until the late 1970s. Now it is central.