The density of human constructions such as residences, commercial buildings, roads, bridges, and trains is high in urban regions. The phrase "urban area" can apply to towns, cities, or suburbs. An urban area comprises both the city and its surrounding territories. The term "rural area" describes places without a significant population center, such as farms, forests, and vacant land. Rural areas are often defined by their relationship to large cities. A rural town is generally described as having between 1,000 and 5,000 people, while a small rural city has about 6,000 people or more. Between these two types of areas lies the category of mid-sized cities. Mid-sized cities are generally considered to have populations between 50,000 and 200,000 people. Large cities are associated with populations of 500,000 or more; while metropolitan areas are defined as having populations greater than 250,000. Small towns and rural villages typically have populations under 10,000 people. However, there are exceptions for churches, schools, and other institutions that cluster together form a new type of community. These clusters can have many thousands of residents. Urban areas cover 70% of the earth's surface, but they include only 30% of the population. Most countries are divided into urban and rural areas.