CN Tower
"Guinness World Records" has called the CN Tower "the world's tallest self-supporting tower" and "the world's tallest free-standing tower". Although Guinness did list this description of the CN Tower under the heading "tallest building" at least once, it has also listed it under "tallest tower", omitting it from its list of "tallest buildings." In 1996, Guinness changed the tower's classification to "World's Tallest Building and Freestanding Structure". Emporis and the Council on Tall Buildings and Urban Habitat both listed the CN Tower as the world's tallest free-standing structure on land, and specifically state that the CN Tower is not a true building, thereby awarding the title of world's tallest building to Taipei 101, which is shorter than the CN Tower. The issue of what was tallest became moot when Burj Khalifa, then under construction, exceeded the height of the CN Tower in 2007 (see below).
List of tallest buildings in Philadelphia
Philadelphia has twice held the tallest habitable building in North America, first with Christ Church, then with City Hall. The latter reigned as the world's tallest building from 1894 to 1908, and is currently the world's second-tallest masonry building, only shorter than Mole Antonelliana in Turin. Like other large American cities, Philadelphia went through a massive building boom in the 1970s and 1980s, resulting in the completion of over 20 high-rise buildings.