A model of the flows of bilateral trade based on analogy with the law of gravity in physics: Tij = AYiYj /Dij , where Tij is exports from country i to country j, Yi,Yj are their national incomes, Dij is the distance between them, and A is a constant. Other constants as exponents and other variables are often included. Due independently to Tinbergen (1962) and Pöyhönen (1963).