A model of the flows of bilateral trade based on analogy with the law of gravity in physics: T_ij = AY_iY_j /D_ij , where T_ij is exports from country i to country j, Y_i,Y_j are their national incomes, D_ij 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).