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).
History of the world's tallest buildings
From the 13th century until 1894, the world's tallest building was always a church or cathedral. Old St Paul's Cathedral with its spire was completed in the 13th century. The central spire of Lincoln Cathedral surpassed Old St Paul's in the early 14th century. The Lincoln Cathedral's spire collapsed in 1549, beginning a long interval where the status of world's tallest building was borne by shorter buildings. St. Mary's Church in Stralsund became the world's tallest building after the collapse of Lincoln Cathedral's spire. The central tower of St. Pierre's Cathedral was tallest from 1569 until it collapsed in 1573, making St. Mary's the tallest once again. In 1647, the bell tower of St. Mary's burned down, making the shorter Strasbourg Cathedral the world's tallest building.