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
Before Burj Khalifa was completed, Willis Tower led in the height-to-tip category with after its antenna was extended in 2000, making Willis Tower slightly taller height-to-tip than the WTC north tower's antenna that measured . After the September 11, 2001, attacks, the WTC became the world's tallest two buildings to be destroyed or demolished. They took that distinction from the Singer Building, which stood tall until the late 1960s where One Liberty Plaza now stands right across Church Street from the WTC site.