1/3. For better or worse, high-rise structures have transformed the world's cities, with skyscrapers a particularly potent symbol of a booming economy. Alan Yentob discovers how advances in technology in the early 20th century - combined with rivalry, wealth, power and ego - fuelled spectacular construction projects in New York and Chicago, including the Art Deco Chrysler Building, the Chicago Tribune tower competition and the iconic Empire State Building.

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