A structure or part of a structure that is exceptionally tall in proportion to its base dimensions. Towers may be made of steel, reinforced concrete, or timber and are constructed for many purposes, such as to support electric power transmission lines, radio and television antennas, windmills, and rockets and missiles prior to launching (Fig. 1). Skyscrapers are often not classified as towers, although they are sometimes called towers. For example, the former World Trade Center buildings in New York City were also known as the Twin Towers. In some countries, such as India and the United Kingdom, tall residential buildings are called tower blocks.