1. The empirical regularity observed by Thirlwall (1979) that for many countries the rate of growth of output, g_Y, is approximated by the rate of growth of exports, g_X, divided by the country's elasticity of demand for imports, η_M: g_Y = g_X/η_M. 2. Equivalently, letting export growth be driven by foreign income growth, g_Y*, and the elasticity of (foreign) demand for exports, η_X, this equates the ratio of foreign and domestic growth rates to the ratio of the trade elasticities: g_Y/g_Y* = η_X/η_M. The latter was dubbed by Krugman (1989) the "45-degree rule. "