I study one of the most profound cultural changes of the 20th century: the rise of casual dress. I study casual dress as it evolved on the beaches of Miami. I study casual dress as worn by the Black Panthers and by Princeton undergraduates. As a professor, I teach seminars on material culture and direct graduate students as they research and curate costume exhibitions, but my bread-and-butter as a scholar is the “why” and “when” our sartorial standards went from collared to comfortable.

I happen to own 17 pairs of sweatpants, but I am a convert to casual. As a teen, I scoffed at the wrinkled khakis of my high-school colleagues and scoured the thrift stores of central Pennsylvania in search of the most non-casual clothes I could find—wasp-waist wool dresses, opera gloves, and evening bags. By my mid-20s, I realized I no longer wanted to pry my 6-foot-tall body into uncomfortable clothes and stay in them for hours. While my Clergerie-clad best friend chased down taxis and potential husbands in 3-inch heels, I chose cowboy boots and a pair of overalls that same friend said made me look like an oversized baby. For me, casual is not the opposite of formal. It is the opposite of confined.

As Americans, our casual style uniformly stresses comfort and practicality—two words that have gotten little attention in the history of fashion but have transformed how we live. A hundred years ago, the closest thing to casual was sportswear—knitted golf dresses, tweed blazers, and oxford shoes. But as the century progressed, casual came to encompass everything from worker’s garb (jeans and lumberman jackets) to army uniforms (again with the khakis). Americans’ quest for a low-key style has stomped on entire industries: millinery, hosiery, eveningwear, fur, and the list goes on. It has infiltrated every hour of the day and every space from the boardroom to the classroom to the courtroom.