If you had to name the one item of clothing that most defines American style it would be the blue jean. Blue denim existed—stiff tough cotton—but the blue jean was born in gold-rush San Francisco when Levi Strauss & Co. patented the use of metal rivets—right, rivets!—to increase the durability of denim work pants. A new exhibition tracks the evolution and endless permutations of the all-American blue jean, from the mythic American West up to this very minute. —J.V.