Chief engineer Fazlur Khan and SOM architect Bruce Graham decided that the Sears Tower would have nine tubes bundled together. Each tube would be a 75 foot (23 m) square. All the tubes would rise to a height of forty-nine stories. Three tubes would end at this height. The rest would end at higher floors. Only two would rise to the full height of ninety-five stories. With this design, the floor space decreases as the tower rises. Sears could use the larger lower floors. The company could rent the smaller upper floors until it needed them. The outside open spaces created where tubes ended are called setbacks. The setbacks gave the tower a unique look. When the steel tubes and trusses were finished, the building was covered in black aluminum and dark-tinted windows. This type of smooth exterior is called a curtain wall. Curtain walls add to a skyscraper’s upright look.