In central places theory, the k value is often used to define the geographical relationship between different orders. With a k=3 relationship, each market area of a higher-order contains three market areas of a lower order. Several other values of k are possible in regional representations of urban hierarchy, but the most common are 3, 4, and 7:
- The marketing principle (k=3). The territory is covered by a minimum number of urban centers. Each center has three options to purchase goods and services of a higher order.
- The transport principle (k=4). In this distribution, as many centers as possible are along main transport lines. The system tends to be linear in orientation, which minimizes the distance between each settlement. With the transportation principle, towns not on major transportation routes are smaller than expected. Transportation routes attract business and allow more large towns to develop, such as for those along a railroad.
- The administrative principle (k=7). The central place system is organized in such a way that there is a clear separation of all market areas. In the k=3 and k=4 principles, the border between market areas of a center of higher order is composed of lines between centers of lower order. Administratively, it does not make sense since all towns are part of a specific administrative division. According to this principle, the boundary of administrative divisions is located halfway between two centers of the same order.