Population growth and the increasing density of cities created a housing crisis in the 20th century. The apartment building emerged as a solution for housing large numbers of people in small areas. Although a preexisting type, during the 20th century the development of the apartment building dramatically reshaped the built environment of cities and their surrounding suburbs. Apartment buildings developed in locations convenient to transportation networks and services that encouraged dense residential land use. The increase in apartment living subsequently inspired continued international dissemination of the modern apartment building type. An apartment building contains multiple dwelling units of one or more rooms. Other basic aspects of the 20th-century apartment building’s program are a bathroom and kitchen for each unit and the provision of heating, ventilation, air conditioning, and other systems. As with other commercial building types, efficient use of space is integral to good a...