Complexity stems from the presence of a relatively stable structure upon which interchangeable elements can be affixed. The 7 properties that account for the existence of cities and their diverse configurations: production, proximity, reproduction, capitalization, place, governance, environment. Refers to the need for cities to produce goods and services for their own residents. Rural communities are traditionally self-reliant, historically cities are unable to satisfy all its consumption requirements. Cities must export to generate revenues in order to buy products from outside their territory. The existence of a city depends on it being able to export sufficient goods/services to counterbalance its imports. Cities that fail to export decline and turn into ghost-towns. Goods/resources drawn from outside a city"s territory increasingly originating from foreign countries and continents in the era of globalization. At the same time, growing emphasis on local sustainability with increasing energy costs and growing environmental awareness.