970; Beineke and Wilson 1978; Wilson and Beineke 1979; and the historical survey by Biggs et al. 1976). Methods for describing and analyzing the structure of networks should be an essential part of network economics. Regulators need to understand the design of communications networks and the effects of regulatory policy on network archi- tecture and performance. In this section, we examine network structures that will be useful in our discussion. 1. Describing Networks A network is a general concept used to describe, among other things, systems of communication, transportation, and distribution. A network is composed of two types of objects: nodes and links. Nodes are the junctions of the network, and links are the connections between the nodes. In graph theory, a network is defined formally as a system of nodes and links, with particular numerical values, such as costs or capacities, assigned to the links. In what follows, we employ two different uses of the term "network" that should be readily apparent from the context. We apply the graph theory meaning of the term "network" to define a generic communications system composed of nodes and links with particular properties of the links. We also apply the economic meaning of the term "network" to indicate a privately owned communications facility provided by a firm. When speaking of network access, we mean the market transactions of a firm that provides a privately owned network. A node in a network can be a terminal point such as the end of a subway line, or it can be a point of origin such as a subway's central station. A node can be simply a point along the line, such as a subway station. Even more simply, a node can be a point in a network where two or more lines reach a juncture. This can be a point in an electrical system where a line bifurcates. Thus, a node can be a point at which two links come together without any type of additional equipment or facilities. A node also can be something much more complex: it can be the location at which one or more pieces