Mr. Porter has a stake in getting people into fancy sweats. It sells versions that can easily top $800. But it does highlight how dress codes keep sliding toward informality in countries such as the US.

It wasn’t long ago that jeans were unacceptable in the office. Now they’re practically business attire, especially in the eyes of kids. CEOs, particularly in Silicon Valley, often dress like it’s the weekend all the time. These shifts are forcing even conservative workplaces to loosen up. JPMorgan Chase, for instance, recently relaxed its policy on office attire. Sweats still aren’t allowed, but jeans are (paywall) with a manager’s approval.

As jeans have gotten more formal, the measure of what’s acceptable has changed, creating room for an even more casual option, though only to a degree. “I think sweatpants still retain a bit more of the stigma that denim has shed,” Emma McClendon, an assistant curator at the Museum at FIT and author of Denim: Fashion’s Frontier, said in an earlier interview. “Sweatpants are not necessarily as acceptable at the workplace, at the opera, at church.”

A model presents a creation from the Burberry Spring/Summer 2016 collection during "London Collections: Men" in London, Britain June 15, 2015. "London Collections: Men" is a four-day showcase of men's fashion scheduled a month before London Fashion Week. REUTERS/Suzanne Plunkett - RTX1GKVF
Image: Reuters/Suzanne Plunkett

Designers love to play with these boundaries. Brands such as Vetements and Haider Ackermann have shown sweats on the runway, and for its spring 2016 men’s collection, Burberry even paired tailored joggers with more formal clothes, such as a shirt and tie. Would the look work for the office? Quite possibly.

Sweats still have their limits though. “Use your judgment wisely,” Mr. Porter warns of its sweats-and-blazer combo. “This is probably not the outfit for that big client meeting.”

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