Before World War II jeans were only worn in America's Western states. In the east they were synonymous with romantic notions of the cowboy - rugged, independent and American, but at the same time rural and working class.
Affluent easterners would escape humdrum suburban life to holiday on "dude ranches" - working farms where they could play at being cowboys - and wearing jeans was part of the experience.
"It was the kind of clothing that represented the American West and it was this cachet and this sort of magical thing," says Lynn Downey, archivist and historian at Levi Strauss & Co. But back in Connecticut or New York, the jeans stayed in the closet.
When they did start to be worn as casual wear, it was a startling symbol of rebellion - the spirit captured by Marlon Brando in his 1953 film The Wild One and by James Dean two years later in Rebel Without a Cause.
"If you were a 15-year-old boy in 1953 you wanted to be Marlon Brando," says Downey.
"Hollywood costume designers put all the bad boys in denim."