FORTUNE -- Dear Annie: Please settle an argument. I work with a brand-management team of about 20, some of whom I really think take the notion of "business casual" clothing a few steps over the line. The question is, where is the line? I say acid-washed jeans (with holes), yoga pants, and tank tops just don't belong in the office. Not that everyone has to go back to the old days of wearing suits and ties all the time, God forbid, but it doesn't help a person's career prospects if they look like they just rolled out of bed, either.
Several of my coworkers, who show up wearing all kinds of strange and sloppy things, disagree. Their argument is two-fold: First, if your work is really topnotch, it doesn't matter what you wear, you're still a star. And second, if Mark Zuckerberg can show up on the floor of the New York Stock Exchange for Facebook's (FB) IPO wearing jeans and a hoodie, it proves there are no more rules about clothes. What do you say? -- Fashion Fan
Dear F.F.: Interesting question. Slowly, over the past 15 years or so, startups' lack of office dress code has permeated the work world. "It started with 'casual Fridays' and devolved from there," says Lauren A. Rothman, author of a new book called Style Bible: What to Wear to Work.
Rothman is a longtime professional fashionista whose consulting firm, Styleauteur, runs dress-for-success seminars at Fortune 500 companies and elsewhere. She's frequently called in to coach individual executives on sprucing up their personal style. "For instance, I've had law firms and major accounting firms call me and say, 'Can you work with So-and-So? He's not going to make partner until he starts looking like one,'" she says.
There's a reason for that: "We'd all love to believe appearances don't matter, but the reality is, packaging counts. What you wear is part of your overall personal brand, your professional image. If you want to move up in your career at almost any big company, you have to look the part. The old adage 'dress for the job you want, not the job you have' is still true."
In other words, if your boss and other higher-ups aren't coming to work in acid-washed jeans and tank tops, your colleagues might want to take the hint. And as for what Mark Zuckerberg wears, unless your coworkers also happen to be self-made billionaires, how relevant is that?
Some uncertainty over how to dress for work springs from the fact that, although many companies do still have actual dress codes -- which tend to vary a lot from one industry, and one region of the country, to another -- "managers at most businesses don't do a great job of communicating what is expected, or what image the company wants employees to project," Rothman notes. "It's hard to hold people to a standard if you haven't told them what the standard is. Nevertheless, most employers do want you to dress differently than you would if you were just hanging out at home."
In practical terms, that means that, for anyone who wears jeans to the office, "a professional look is still a good idea. Not just any old jeans will do," Rothman says, adding that office denim should be a dark wash, hemmed so they just brush the tops of your shoes, "not fashionably dragging on the floor, and free of rips, whiskering, or anything else that marks them as overly trendy, or old. Look for trouser-style jeans whose cut resembles dressier pants."
Style Bible goes into lots of lively detail about how to put together a work wardrobe, depending on your job, your budget, and where you live. In general, Rothman says that some mistakes women make revolve around "the sexiness factor -- wearing too-short skirts, too-high heels, or too much makeup." For men, she's most often called upon to help address sloppiness, including "stained or wrinkled clothes, or clothes that don't fit properly."
Your coworkers who think clothes don't matter might want to consider a couple of further thoughts. First, Rothman notes that advancing a career these days depends in large part on networking. So "even if you believe that the quality of your work should speak for itself, what about the way you come across to people who aren't yet familiar with how good your work is?" she says. "If you're going to networking events, people there are forming first impressions of you based in part on how professional you look." Until they've gotten to know you, they have little else to go on, so it's smart to make sure your style isn't getting in the way.
And second, Rothman suggests that those who doubt that clothes matter conduct a small experiment: "Just try dressing more professionally for a week, or a month. Most of us feel more confident and more competent when we dress well. You may even find that other people respond to you differently. It can be hard to command attention and respect when you look as if you just don't care.
"You have to get dressed every day anyway -- you can't go to work naked," she adds. "So why not try to make what you're wearing work for you?" Why not, indeed.
Talkback: Do you think "business casual" has gone too far, where you work? If your company has a dress code, what is it? Leave a comment below.
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