A skyscraper is a continuously habitable high-rise building that has over 40 floors and is taller than approximately 150 m (492 ft). Historically, the term first referred to buildings with 10 to 20 floors in 1880s. The definition shifted with advancing construction technology during the 20th Century. Skyscrapers may host commercial offices or residential space, or both. For buildings above a height of 300 m (984 ft), the term "supertall" can be used, while skyscrapers reaching beyond 600 m (1,969 ft) are classified as "megatall". One common feature of skyscrapers is having a steel framework that supports curtain walls. These curtain walls either bear on the framework below or are suspended from the framework above, rather than resting on load-bearing walls of conventional construction. Some early skyscrapers have a steel frame that enables the construction of load-bearing walls taller than of those made of reinforced concrete. Modern skyscrapers' walls are not load-bearing, and most skyscrapers are characterized by large surface areas of windows made possible by steel frames and curtain walls. However, skyscrapers can have curtain walls that mimic conventional walls with a small surface area of windows.
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