A new shopping development in one part of town might well flourish, but often at the expense of older established centres elsewhere. It is vital, therefore, that some coordination is exercised and, to this end, planners have developed increasingly elaborate techniques with which to approach retail planning. In operational shopping models, analysts have tended to use three particular measures as proxy variables for attractiveness: floorspace, sales, or some index of the type/mix/number of shops. A fuller understanding of the application of the constrained gravity model in shopping studies is, perhaps, best gained by demonstrating its use with reference to a simple example. Empirical studies made in the 1940s by Zipt (1949) and, independently, by Stewart (1947), shows that a number of human activities involving interaction over space could be predicted, with surprising accuracy, by the laws used in Newtonian physics. The gravity and potential concepts of human interaction were originally developed from analogy to the Newtonian physics of matter.