Hubs are special nodes that are part of a network linked by spokes, and located in such a way as to facilitate connectivity between interacting places. See O’Kelly (1998) J. Transp. Geog. 6, 3, 171. Song and Ma (2006) Chinese Geogr. Sci. 16, 3, 211 show how commercial and operational freedoms have led most of the larger airlines to develop hub-and-spoke networks.