Inspiring, coaching, critiquing, training, promoting, problem-solving. There is no shortage of work to be done by good managers.

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.