An interlocking network model developed at GaWC. ‘In central flow theory, flows generate a network; a space of flows. Central flow theory produces a spatial network of urban settlements’ (Taylor (2007) GaWC Res. Bull. 238). City networks are constituted by the interlocking of cities by commercial agents in the everyday course of their business practice; central flow theory entails network mutuality, and this suggests that urban policy makers should focus on cooperation with other cities.