World cities are generally deemed to form an urban system or city network but these are never explicitly specified in the literature. In this paper the world city network is identijied as an unusual form of network with three levels of structure: cities as the nodes, the world economy as the supranodal network level, and advanced producer service firms forming a critical subnodal level. The latter create an interlocking network through their global location strategies for placing ofices. Hence, it is the advanced producer serwice firms operating through cities who are the prime actors in world city network formatwn. This process is formally specified in terms of four intercity relational matrices-elemental, proportional, distance, and asymmetric. Through this specification it becomes possible to apply standard techniques of network analysis to world cities for the first time. In a short conclusion the relevance of this world city network specification for both the0 y and poli y-practice is briefly discussed.The contemporary study of world cities can be said to begin with Friedmann and Wolffs (1982) identification of "command centers" to control and articulate the "new international division of labor" being created by multinational corporations. They introduced the concept of a "global network of cities" (King 1990, p. 12) performing these functions and this idea has persisted to the present: for instance, Sassen's (1994, p. 47) "transnational urban system," Lo and Yeung's (1998, p. 10) "functional world city system," and Short and Kim's (1999, p. 38) "global urban network." Many other similar phrases could be listed from the literature but the key point is that they all have one shared characteristic, the failure adequately to specify the system or network. Castells (1996) does theoretically incorporate Sassen's ideas as part of the middle layer in his "space of flows," global cities as "the most direct illustration" of his "nodes and hubs" (p. 415), but he does not provide any additional specificity to this global network (p. 413). The general purpose of this paper is to supply just such a specijkation.The need for a precise specification of the world city network is obvious. Without it there can be no detailed study of its operation-its nodes, their connections, and how they constitute an integrated whole. Smith and Timberlake (1995, p. 85) have recognized this point but offer only a taxonomy of flows before calling for the use of net-

Central place hierarchies have been the traditional basis for understanding external urban relations. However, in contemporary studies of these relations, a new emphasis on urban networks has emerged. Rather than either abandoning or extending central place thinking, it is here treated as representing one of two generic processes of external urban relations. Town-ness is the making of ‘local’ urban—hinterland relations and ‘city-ness’ is the making of ‘non-local’ interurban relations. Central place theory describes the former through an interlocking hierarchical model; this paper proposes a central flow theory to describe the latter through an interlocking network model. The key difference is the level of complexity in the two processes.

Drawing on previous speci cation and measurement of the world city network, this paper develops an exploratory research design using principal components analyses. Multiple solutions are used to explore the structure of a matrix de ned by 123 cities and 100 global service rms in which the cells indicate the value of the rm's of ce in that city. Principal components analysis produces clusters of cities based upon similar pro les of service rms. Using 13 separate analyses, the main nding is a prime structure consisting of Outer cities, US cities, Paci c Asian cities, Euro-German cities and Old Commonwealth cities. Other secondary structures are also found. The end-result is a new geography of globalisation as indicated by con gurations of the world city network.

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