COLORADO SPRINGS, Colo. - Brook Mark doesn't care about numbers.

Give her a 4, 6 or 8. Her closet has them all.

For most women, lower is better when talking about clothes. But Mark knows not to let a numbers game fool her. She's fully aware that the true size of her clothing largely depends on the brand.

"I try to buy the same misses sizes, and I can't do that anymore," says Mark, a master gardener who lives in Colorado Springs, Colo.

Women's fashion has always had its own rules, and the rule when it comes to sizing clothes is this: Things aren't always as they seem. One brand's 12 might be the same size as another brand's 8.

It's called "vanity sizing" - putting smaller numbers on bigger clothes. As American waistlines have grown, companies have realized women will spend more money for a smaller number, leading to the sizing rule of thumb: the more you spend, the smaller number you'll wear. But even mainstream brands have taken hold of the concept and started peddling the idea to the average mall shopper.

Which is why Mark, who wears brands such as Lee, Wrangler and Coldwater Creek, doesn't wear just one size anymore.

"I don't care about the number, I care about how it fits," Mark says. "But the standard numbers that I used are no longer applicable."

Why? Well, as the name implies - vanity. Shopping is an emotional experience for women, and marketing firms have caught on, says Cheryl Locke, fashion journalism coordinator for the School of Fashion at the Academy of Art University in San Francisco.

"I think it has to do with feeling beautiful - feeling magically transformed by what you're wearing," Locke says.

To amp up that feeling, some brands have even revamped their entire sizing system - such as Chico's, which did away with the traditional 6, 8, 10, 12, 14 concept from the start in lieu of one that sizes clothing from 0 to 3.

The thought being: Smaller is always better.

"They know they can hook that woman because that woman knows they're a 1," says Shelley Laur, owner of Swish, a secondhand clothing store in Colorado Springs, Colo. "I don't know if the media has done that, or if we just buy into that because a smaller number is a smaller size and it just kind of sticks that way in your brain."

She continues, "I know a lot of women won't even try on (bigger sizes) - if I know a woman is a 10, they refuse to try on a 10. They keep trying on 8s."

Companies know they can hook a customer with a smaller size, which is why the fashion industry has lagged to return to standard sizing, Laur says.

Sizing used to be standardized in the `50s, when more women began buying clothes instead of making them, says Laur, who specializes in retro and vintage clothing. But vanity sizing soon started to play a role - largely in the `60s and `70s - resulting in the Department of Commerce officially withdrawing commercial sizing standards in 1983. Now only pattern companies use these measurements, according to the National Institute of Standards and Technology Museum Web site.

"Before, they used to size things by bust," Laur says.

To see vanity sizing in action, just take a look back at the sizing of yore. Marilyn Monroe, whose voluptuous body required a Size 16 in the `50s, was actually more of a 6/8 by today's standards, Laur says. Generally speaking, clothing sized in the 1950s can be cut in half for an idea of today's mainstream sizing.

In an attempt to get back to more standardized sizing, the Textile Clothing Technology Corp. invented a body scanner to collect measurements that could help devise a uniform scale. "Size USA," a 2003 study conducted by the company, scanned 6,310 American women and found that the average waist size varied between 32.6 and 37.4 inches, depending on age and race.

Still, when talking about average sizes, it's hard to say what that means in terms of clothing tags. Average typically refers to a Size 12, which Locke describes as a "Banana Republic 12." Banana Republic's fit guide says a Size 12 pant is 30½ at the natural waist.

Couture clothing has stayed more consistent with sizing - largely because the number is just as important as the tag. It's hard to find designer brands above a Size 8, Locke says. Even XL pants from Italian designer Roberto Cavalli are vanity sized at 30½ at the waist - the equivalent of a Banana Republic 12.

"High fashion does become scary for people who aren't a certain size," Locke says. "That's why I think companies like Liz Claiborne, Target and Gap - they continue to inflate their sizes - then the American woman is comfortable shopping there. It's a very psychological phenomenon."

So how can you beat the system? Go for fit and forget about numbers. (Your secret's safe with the saleswoman.) Your best bet is to try things on before buying, but that's not always possible.

"Most people don't like to have to try things on - they don't have time," Mark says.

In which case, make sure the stores you frequent have friendly return policies.