New York ? When you’re working with Madonna and Missy Elliott, you never know what you’re going to get — and that’s just what Gap wanted.

For its new fall ad campaign, the retailer gave each of the two music and fashion icons a pair of corduroy pants and a couple of tops with instructions to customize the garments any way they wanted.

Madonna had “Lady M” silkscreened down one of the legs of her light-blue, low-rise cuffed cords, pairing the pants with a Lana Marks alligator belt and her own newsboy hat; Elliott decorated her pants with studs, attached a gray hoodie sweatshirt to a corduroy jacket and had an airbrushed portrait of herself added to a plain white T-shirt. Each also had a big M embroidered onto her back pocket.

“The essence of each of their styles is so different, but both these women emphasize individuality and uniqueness. And both look good in regular cords and T-shirts — just like everyone else,” says Joe Zee, W magazine’s fashion director who styled the print and TV ads.

Aside from telling them the theme was corduroy, Gap allowed Zee to work directly with Madonna and Elliott to devise the look of the ads, he says. It really was a coincidence that both women chose the same color pants.

“It was never dictated, it was a collaboration from Day 1. … Both Madonna and Missy liked the blue because it had a vintage feel,” Zee says.

Was it weird to see such glamorous women in the clothes that the rest of us wear to the grocery store?

“This is what they (Madonna and Elliott) wear. Missy gets on an airplane dressed like this, and I definitely think Madonna wears a tank top and short cords on the weekend,” Zee responds.

But for a little extra sparkle, Elliott brought her own monogrammed jewels to the Gap photo shoot, while Madonna wore a borrowed diamond wallet chain by Neil Lane and several diamond bracelets, together valued at $5 million.

“The diamond wallet chain is very street, hip and urban, but it’s Madonna, so we wanted it to be a little more glam,” Zee says.

Music and fashion icons Missy Elliott, left, and Madonna star in Gap's fall advertising campaign. They both customized corduroy pants -- and the rest of their look -- to suit their individual styles.

Rebecca Weill, director of public relations for Gap, says the company sought out Madonna and Elliott because they are fashion fans who are willing to take chances.

“We knew we wanted the campaign to have an optimistic youthful energy, something that would appeal to our more adventurous customers,” she explains.

(Those adventurous Gap customers get their chance to embroider their own cord pants on Gap.com, while supplies last.)

Madonna, 44 and mother of two, has maintained her position as the ultimate fashionista for 20 years because of her instincts and her willingness to change.

“I don’t think she consciously decides, ‘Today I’m going to be blond and rock the world’ or, ‘Today I’m going to be a brunette geisha and rock the world,”‘ says Zee. “She just has such a strong instinct about what’s going and what she feels is right, and she just runs with it.”

The September issue of Harper’s Bazaar celebrates 20 years of Madonna style, complete with a timeline tracking her various “looks,” including 1983’s “dance queen,” the “Evita” style of 1996 and the “rhinestone cowgirl” of 2000.

“Twenty years on, the look she’s presenting right now is pretty similar to how we first saw her,” observes Bazaar editor in chief Glenda Bailey. “The difference is the diamonds are now real.”

Bailey says women admire Madonna in a way they don’t most other red-carpet stars because Madonna knows what works for her — and what doesn’t. She won’t embrace a trend just because it’s there, she adds.

Madonna also teaches a valuable lesson, says Bailey: “We’re born with a particular look, but it’s empowering to know you can completely transform yourself into who you want to be.”