Domestic Trade Domestic trade occurs automatically between cities of the same civilization that are connected to each other. Any tradable resource within your territory that is connected to any city is shared by all connected cities. Every city that is connected to a resource has that resource in the appropriate box of the City Display. Foreign Trade A single resource icon is enough to supply the needs of an entire civilization. Excess resources may be traded to other civilizations if the capital cities of both civilizations are connected. You trade strategic and luxury resources with other civilizations during diplomatic negotiations. Trade Route Connections Two cities are connected if any one of the following are true: * there is a road or railroad running between them, * there are harbors in both cities and a visible water route between them, * or both cities contain an airport. Water Routes: In order to use a water trade route, your civilization must be able to safely traverse every square of the route and this depends upon having Map Making, Navigation, and/or Magnetism. The only exception involves enemy territory: two cities are not connected if the only road/rail that connects them passes through enemy territory, or if a harbor is blockaded by enemy naval forces.