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 and structures
The height is measured from the level of the lowest, significant, open-air, pedestrian entrance. At the time, the Willis Tower held first place in the second and third categories, the Petronas Towers held the first category, and the original WTC north tower held the fourth (height to tip) category with its antenna. In 2000, however, a new antenna mast was placed on the Willis Tower, giving it the record in the fourth category. On April 20, 2004, the 101-storey Taipei 101 in Taipei, Taiwan, was completed, taking the world record for the first three categories. On July 21, 2007, it was announced that Burj Khalifa in Dubai, UAE, had surpassed Taipei 101. Since its completion in early 2010, Burj Khalifa leads in all categories (the first building to do so) with its spire height of .