3.1 Introduction Although many of the interactions considered in Chapter 2 were unrestricted in the sense that they could flow freely in any direction, most movements are constricted in some sort of channel. Even air-routes are, as Warntz (1961) has shown for the North Atlantic, confined to most movements flow corridors; along fixed channels-roads, pipelines, telephone wires. These features themselves pose distinct locational problems which are regarded here as part of a general class of network Network has literature problems. location a which includes some classic early studies (eg Lalanne, 1863) but it is a topic which has been strangely neglected in standard treatments of locational theory. It represents a area of common interest for both human geography and physical (Haggett Chorley, 1969). geography and Here we develop some of the elementary spatial models of network more structure relating to location, density, and change over time. Readers are referred to Chapter 9 (see especially 9.7) description of measures Section for a of network structure.