When asked what men can do to improve women’s lives at work, Mary Barra gets straight to the point: “Stop making assumptions,” she tells Quartz.

As chief executive at General Motors, Barra practices what she preaches. Her management philosophy is epitomized by GM’s workplace dress code—which is equally brief, and also an antidote to the restrictive, wallet-draining policies at many large corporations. It reads, in full: “Dress appropriately.”

Having worked for GM since she was a teenager—first as a factory-floor inspector, then scaling through leadership roles in engineering and communications—Barra was well-acquainted with the automaker’s bureaucracy by the time she became vice president of global human resources in 2009, months after the company filed for bankruptcy.

Instead of immediately focusing on high-level restructuring strategy, Barra surprised her colleagues by tackling the small, seemingly inconsequential policies she knew were foundational to company culture. Her first battle: The dress code.

She told the story at the Wharton People Analytics Conference, held in Philadelphia on March 23:

“A lot gets set aside when you’re going through a restructuring process, so it was an opportunity to really define our culture. So, brainstorming with the HR department, I said let’s change the dress code. Let’s make it ‘dress appropriately.’

But the HR department ironically posed my first hurdle. They started arguing with me, saying, it can be ‘dress appropriately’ on the surface, but in the employee manual it needs to be a lot more detailed. They put in specifics, like, ‘Don’t wear T-shirts that say inappropriate things, or statements that could be misinterpreted.'” 

Barra was perplexed.

“What does inappropriate, in the context of a T-shirt, even mean,” she asked the audience, half-jokingly. ”So I finally had to say, ‘No, it’s two words, that’s what I want.’ What followed was really a window into the company for me.”

After replacing GM’s 10-page dress code treatise with a two-word appeal, Barra received a scathing email from a senior-level director. ”He said, ‘You need to put out a better dress policy, this is not enough.’ So I called him—and of course that shook him a little bit. And I asked him to help me understand why the policy was inept.”

The director explained that occasionally, some people on his team had to deal with government officials on short notice, and had to be dressed appropriately for that. 

“Okay, why don’t you talk to your team,” Barra replied. ”He was an established leader at GM, responsible for a pretty important part of the company, with a multimillion-dollar budget. He called me back a few minutes later, saying, ‘I talked to the team, we brainstormed, and we agreed that the four people who occasionally need to meet with government officials will keep a pair of dress pants in their locker. Problem solved.'”

Simple as it seems, this was a big “a-ha” moment for Barra as a leader, and for GM at large.

“What I realized is that you really need to make sure your managers are empowered—because if they cannot handle ‘dress appropriately,’ what other decisions can they handle? And I realized that often, if you have a lot of overly prescriptive policies and procedures, people will live down to them,” she said.

“But if you let people own policies themselves—especially at the first level of people supervision—it helps develop them. It was an eye-opening experience, but I now know that these small little things changed our culture powerfully. They weren’t the only factor, but they contributed significantly.”

While GM’s dress code empowers all employees, it’s particularly impactful for women. As Quartz’s Marc Bain has noted, while men can easily repeat suits and slacks, for women, dressing for work has never been more confusing than it is today.

“Conservative sectors, such as finance and law, may be slowly loosening up, but they still often require fairly formal clothing. Silicon Valley, meanwhile, is a bastion of informality, home of the business hoodie. In between those two poles are any number of offices that fall at different points along the corporate-to-casual spectrum. ‘Work clothes’ no longer just means suits, blazers, starched shirts, and tailored trousers. The situation can make it difficult for anyone to get a handle on what is and isn’t right for the office.”

Though more guidance from labels and fashion media might be warranted, prescriptive dress codes from employers, at their core, represent institutional fear that workers can’t (or won’t) present themselves respectfully. This is a paternalistic assumption to make of any willfully employed adult—but it’s particularly damning when that employee is a woman, and the rule-setter is a man who’s never spent a day in heels or a pencil skirt.

By simply stating “dress appropriately,” Barra does exactly what she asks of other leaders: She avoids assumptions, instead choosing to trust her employees’ judgment—and has found the experience remarkably liberating.

This story is part of How We’ll Win, a project exploring the fight for gender equality at work. Read more stories here.