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).
Vanity height
The controversy began when the Petronas Towers were named as the world's tallest buildings in 1998, despite having a roof 63.4 m (208 ft) lower than that of the Willis Tower. The current world's tallest building, Burj Khalifa, is officially 828 meters tall, but its highest usable floor is 585m above ground. Therefore, its vanity height is defined as 244 meters, or 29% of the building's total height. The likely next tallest building, Jeddah Tower (designed by the same architect), will be over 1,000 meters tall but its highest floor is 630m above ground. The top 370m (equivalent to an 85-story building) or 37% of the building's total height is unusable. When vanity height is excluded, the height progression of the world's tallest buildings looks much more modest in comparison.