was still considered the tallest at that time. Excluding their spires, which are higher than the flat roof of Willis, the Petronas Towers are not taller than Willis. At their convention in Chicago, the Council on Tall Buildings and Urban Habitat (CTBUH) found the Willis Tower (without its antennas) to be the third-tallest building, and the Petronas Towers (with their spires) to be the world's two tallest buildings. Responding to the ensuing controversy, the CTBUH then revised their criteria and defined four categories in which the world's tallest building can be measured, retaining the old criterion of height to architectural top, and adding three new categories: Height to Architectural Top (including spires and pinnacles, but not antennas, masts or flagpoles). This measurement is the most widely used and is used to define the rankings of the 100 Tallest Buildings in the World. Highest Occupied Floor Height to Top of Roof (omitted from criteria from November 2009 onwards) Height to Tip The height-to-roof criterion was discontinued because relatively few modern tall buildings possess flat rooftops, making this criterion difficult to determine and measure. The CTBUH has further clarified their definitions of building height, including specific criteria concerning subbasements and ground level entrances (height measured from lowest, significant, open-air, pedestrian entrance rather than from a previously undefined "main entrance"), building completion (must be topped out both structurally and architecturally, fully clad, and able to be occupied), condition of the highest occupied floor (must be continuously used by people living or working and be conditioned, thus including observation decks, but not mechanical floors) and other aspects of tall buildings. 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