The old idea that men should not wear a suit other than as a suit is taking a beating from the current economy.

Concerned that the jacket, if worn separately, might wear out before the pants, or vice versa, suit makers and retailers have encouraged wearing them together.

No more. While the life of a suit is obviously longer if its wear is limited, men are increasingly thinking of their suits as separates -- as women do -- and integrating them into their wardrobes.

This system works well, for example, with the tan poplin suit, a likely successor to the seersucker suit as Washington's warm-weather uniform. Tan poplin and its counterpart, cotton khaki, is enjoying a revival with the return to safe and classic preppy-dom. And the increased demand for natural fabrics -- including all-cotton shirts -- no doubt spills over into khaki appeal. Like jeans, the trousers only get better -- and more comfortable -- each time they're worn or washed.

Khaki as a color is, of course, an unbeatable neutral to combine with all the bright accents showing up in shirts, ties, belts (even socks). Plus, it will not upstage whatever is on top.

A cotton and polyester blend is best, according to Robert Davis, Bloomingdale's (White Flint) men's clothing department manager, since the combination holds up better than all-cotton.

"Sometimes men like the wrinkled look," he says, "but not always, and not for going into the office."

Our scheme for stretching the basic tan poplin suit into spring and summer separates:

Business -- The suit with a classic white button-down shirt, rep stripe or foulard tie and lace-up shoes.

Saturday afternoon -- The jacket as casual sportcoat, with Izod alligator or other knit polo-style shirt in a lavender or berry tone, or -- for those more adventuresome -- in turquoise. White trousers, say in white duck cloth or white cotton chino or gabardine. A contrast belt and Topsiders for added informality.

Social occasions -- except for those requiring black tie or even a dark suit, tan poplin pants with navy blended hopsacking blazer (with center vent and patch and flap pockets). Pastel blue shirt with white collar and perhaps a yellow (burgandy for the more conservative) knit square-bottom tie.Loafers.

This basic wardrobe will, of course, cover a wide price range. As an example, the pivotal tan suit is available at The Designers (by Giorgio Armani, in cotton and line) for $425; at Bloomingdale's for $140, and at Joseph Bank, $90.