• The World Trade Center had two 110-story buildings, known as the "Twin Towers" and five smaller buildings.

• Tower One was 1,368 feet (414 meters) tall

• Tower Two was 1,362 feet (412 meters) tall

• The Twin Towers were the tallest until the Sears Tower surpassed them.

• Architects: Minoru Yamaski & Associates and Emery, Roth & Sons.

• Built of aluminum and steel.

• The foundation of each tower extended more than 70 feet below ground, resting on solid bedrock.

• Constructed on six acres of landfill.

• The towers were the best known examples of "tube buildings," which are strenghtened by closely spaced columns and beams in the outer walls.

• Each tower consisted of 104 passenger elevators and 21,800 windows.

• About 50,000 people worked in the complex, which housed the offices of more than 430 businesses from 26 countries.

• Completed in 1970.

• Automatic window-washing machines cleaned 600,000 square feet of glass.

Source: World Trade Center and PBS