The same qualities that made New York's World Trade Center the ultimate symbol of American capitalism also made it an irresistible target for terrorism.
Its twin towers were tall and slender. Full of people. At the tip of Manhattan. Uncommonly telegenic.
Those traits combined to create a powerful sight as the towers disappeared from New York's skyline yesterday.
"The World Trade Center was a target because it's a symbol of money and power," said Baltimore-based architectural historian Phoebe Stanton. "That's what they're after. They're going to hit us where we hurt."
"It's the visual symbol of the world economic system," said Baltimore architect Jonathan Fishman. "It's the figurative and literal hub of the world economy."
"If a terrorist wanted to demonstrate aggression against capitalism, it's going to be in New York City and it's going to be the World Trade Center," said Lynn Beedle, director emeritus of the Council on Tall Buildings and Urban Habitat, which monitors high-rise projects around the world.
With the World Trade Center's highly visible location and more than 100,000 visitors a day, "it's a place where you can really make an impact - especially a visual one on television," Beedle said. "This was an attack on corporate America."
The towers that collapsed yesterday were the tallest in the world when completed from 1970 to 1973, at a cost of $350 million. Designed by Minoru Yamasaki of Detroit, each rose 110 stories and was 1,350 feet tall - 100 feet higher than the silver mast of the Empire State Building.
Containing 10 million square feet of space, the World Trade Center was surpassed in height in 1974 by Chicago's Sears Tower, which rises 1,454 feet. But it never lost its status as a symbol of America's economic might.
The World Trade Center was built by the Port Authority of New York and New Jersey to promote international commerce. The goal was to centralize the activities of private firms and public agencies engaged in world trade.
Each tower had 104 passenger elevators, 21,800 windows and roughly an acre of rental space per floor. An observation deck was on the 107th floor of the north tower.
About 50,000 people worked in the twin towers, and an additional 80,000 visited on an average day.
The towers are part of a complex - so large that it has its own ZIP code - that also contains a plaza, hotel, underground mall and five other office buildings. They occupy a 16-acre site bounded by Vesey, Liberty, Church and West streets on the west side of Lower Manhattan.
But it was the twin towers that dominated the skyline of lower Manhattan.
Yamasaki, who died in 1986, explored more than 100 building configurations before settling on the concept of twin towers and several lower structures at the base. Among the discarded ideas was a series of lower towers, which drew criticism for looking too much like public housing, and a single tower rising 150 stories, which was considered too big.
The twin towers were designed to be very tall and slender, to maximize the space on the 5-acre plaza. Initially, they were designed to rise 80 to 90 stories. Only later did the architects decide to make them the world's tallest buildings - following a suggestion said to have originated with the Port Authority's public relations staff.
Yamasaki worked closely with structural engineers John Skilling and Les Robertson, and the result is a tower in which design and structure are one. Challenged to design the world's tallest buildings, the engineers came up with an innovative solution: a rigid "hollow tube" of closely spaced steel columns with floor trusses extending across to a central core.
One of the most unusual aspects of the design is that the windows covered only 30 percent of the buildings' surface, as opposed to the virtually all-glass facades of many International Style structures. The columns, finished with a silver-colored aluminum alloy, were nearly 19 inches wide and set only 22 inches apart, making the towers appear to have no windows at all when seen from a distance.
Yamasaki was an unlikely choice to design the World Trade Center. He was better known as a designer of modestly scaled sculptural buildings than skyscrapers.
The close spacing of the windows had as much to do with Yamasaki's morbid fear of heights as with structural and architectural considerations.
Apparently the architect did not feel comfortable unless the floor-to-ceiling windows were narrower than his own shoulder span.