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.