It's difficult to find a garment as widely embraced, worn and loved the world over as jeans. The classic symbol of the American West is now a staple in wardrobes around the world. But why?

Cowboys may wear them but so do supermodels, farmers, presidents and housewives. For some they're comfortable, durable and easy - for others they're sexy and cool. Does this explain their wide appeal? It is a subject that is relatively unstudied, says anthropologist Danny Miller, whose book Blue Jeans will be published next month. In every country he has visited - from the Philippines to Turkey, India and Brazil - Miller has stopped and counted the first 100 people to walk by, and in each he found that almost half the population wore jeans on any given day. Jeans are everywhere, he says. The reason for their success has as much to do with their cultural meaning as their physical construction.

Levi's, as the patented trousers became known, were made in two fabrics, cotton duck and denim. "They found really early on that it was the denim version that would sell," says Paul Trynka, author of Denim: From Cowboys to Catwalks. Denim was more comfortable, softening with age, and its colour gave it a unique character. 

Why did it sell? Because the denim changed as it aged and the way it wore reflected people's lives. Because of its fading quality, denim was sold raw - unwashed and untreated. Not only were they more durable but each pair of jeans began to tell the story of the worker and his work. "Jeans are the most personal thing you can wear," says Miller. "They wear to the body." But the initial explosion of denim into the world of casual wear had more to do with what jeans had come to symbolise.

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.  

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. If you were a 15-year-old boy in 1953 you wanted to be Marlon Brando.

Hollywood costume designers put all the bad boys in denim. They were soon banned in schools from coast to coast which only added to the fervour with which teenagers embraced them. They were affordable and hard-wearing, looked good worn as well as new, and didn't have to be washed often or ironed at all. They conformed to the body in a way that matched even the most finely tailored clothes. This became especially important for women because then, as now, they paid more attention to fit.

This versatility, the ability to become all things to all people, is the secret to jeans' survival as a clothing staple.

In his book, Miller argues they have become so mainstream that they are now a symbol of the ordinary, a garment people put on to feel comfortable and fit in. "Jeans have become this neutral foundational garment. If you want to show you are relaxed, if you want to be relaxed, you wear jeans," says Miller.

But unadorned denim is still special. It's not just a three-dimensional material; it's four-dimensional because it changes with time as well. The eternal appeal of jeans is just that they reflect us and they reflect the lives that we've had in them.