There are 27 natural resources represented by various icons scattered around the world. These fall into three categories, Bonus, Luxury, and Strategic. Bonus Resources give city production bonuses in worked squares within a city radius. Bonus resources cannot be traded to other cities or civilizations. Bonus resources are always visible in explored areas of the map. Luxury Resources also give city production bonuses in squares worked by citizens. But they may also be traded along trade routes, and make content people happy in cities that receive them. Luxury resources are always visible on the map in explored areas. Strategic Resources, like both the others, give bonuses to city production, but, like luxuries they can also be traded. Strategic resources are required for a city to build various military units and city improvements. Strategic resources appear on the map when a Civilization Advance exposes their usefulness. Luxury and Strategic Resources are referred to as Tradable Resources.