All four subway lines cross downtown, forming a quadrilateral configuration, and the Orange and Green Lines (which run approximately parallel in that district) also connect directly at two stations just north of downtown. The Red Line and Blue Line are the only pair of subway lines which do not have a direct transfer connection to each other. Because the various subway lines do not consistently run in any given compass direction, it is customary to refer to line directions as "inbound" or "outbound". Inbound trains travel towards the four downtown transfer stations–Park Street, State Street, Government Center and Downtown Crossing–and outbound trains travel away from these hub stations.