A tower block, high-rise, apartment tower, residential tower, apartment block, block of flats, or office tower is a tall building, as opposed to a low-rise building and is defined differently in terms of height depending on the jurisdiction. It is used as a residential or office building, or has other functions, including hotel, retail, or with multiple purposes combined. Residential high-rise buildings are also known in some varieties of English, such as British English, as tower blocks and may be referred to as MDUs, standing for multi-dwelling units. A very tall high-rise building is referred to as a skyscraper.
High-rise buildings became possible to construct with the invention of the elevator (lift) and with less expensive, more abundant building materials. The materials used for the structural system of high-rise buildings are reinforced concrete and steel. Most North American–style skyscrapers have a steel frame, while residential blocks are usually constructed of concrete. There is no clear difference between a tower block and a skyscraper, although a building with forty or more stories and taller than 150 metres (490 ft) is generally considered a skyscraper.
