This is what fall has become: the world's biggest prep school with all the cool kids dressing a la Mona Lisa smile, Annie Hall and Katharine Hepburn. Herringbone, glen plaid, houndstooth, argyle and tweed will be highlighting this semester, er, season.

"It's going to be a great season for traditional textures and patterns -- most of them from England and Scotland," says David Wolfe, creative director of the Doneger Group, a source for global market trends and merchandising strategies. "Almost all of these patterns are traditional menswear being used for women's wear," he says. It's gender-bending fashion. "It becomes very, very sexy when a women wears a little bit of menswear."

But there are rules.

Don't over-pattern. "It depends on how big one is -- let's be honest," Wolfe says regarding the how-much-pattern question. Try to keep big patterns away from the tush.

Stay away from overplayed designs like pinstripes.

And don't be afraid to cross-reference. Wolfe likes "tweeds mixed with paisley, houndstooth mixed with velvet and very wintry fabrics mixed with light chiffons." But don't go overboard, he says. "It'll give people a headache."

-- Janelle Erlichman Diamond

Clockwise from top left: Raw shirt with argyle sleeves ($286) and Streets Ahead belt with stones ($412) at Denim Bar, 1101 S. Joyce St., Arlington; A. Cheng herringbone jacket, $345 at Circle Boutique, 1608 17th St. NW; pony-skin hobo bag, $400 at Prua, 2601 P St. NW or; Calypso houndstooth jacket, $275 at Wink, 3109 M St. NW; Cynthia Steffe tapestry blazer in apple tweed, $368 at Sitting Pretty, 35 Maryland Ave., Annapolis.