A model of international trade in which comparative advantage derives from differences in relative factor endowments across countries and differences in relative factor intensities across industries. Sometimes refers only to the textbook or 2x2x2 model, but more generally includes models with any numbers of factors, goods, and countries. Model was originally formulated by Heckscher (1919), fleshed out by Ohlin (1933), and refined by Samuelson (1948, 1949, 1953).