An important problem in biology is to explain how patterned neural connections are set up during ontogenesis. Topographically ordered mappings, found widely in nervous systems, are those in which neighbouring elements in one sheet of cells project to neighbouring elements in a second sheet. Exploiting this neighbourhood property leads to a new theory for the establishment of topographical mappings, in which the distance between two cells is expressed in terms of their similarity with respect to certain physical properties assigned to them. This topographical code can be realized in a model employing either synchronization of nervous activity or exchange of specific molecules between neighbouring cells. By means of modifiable synapses the code is used to set up a topographical mapping between two sheets with the same internal structure. We have investigated the neural activity version. Without needing to make any elaborate assumptions about its structure or about the operations its elements are to carry out we have shown that the mappings are set up in a system-to-system rather than a cell-to-cell fashion. The pattern of connections develops in a step-by-step and orderly fashion, the orientation of the mappings being laid down in the earliest stages of development.