1. In a matrix, the elements on a straight line from the top left to the bottom right, or occasionally from the bottom left to the top right. 2. In an Edgeworth box, the straight line from the bottom left corner to the top right. Along the diagonal, the ratios of allocations of the two agents (industries or consumers) are constant and equal. 3. In an integrated world economy diagram, the straight line from the bottom left corner to the top right. Along the diagonal, the ratios of factor endowments of the two coutries are constant and equal.