In a time of escalating clothing costs and fashion ennui, ''modular dressing'' is catching on across the nation. Sold in department and specialty stores by clerks trained as stylists, collections like those from Singles, Multiples and Units promise freedom from the ironing board as well as from the dictates of fashion.

A kind of wardrobe system with interlocking pieces, worn every which way and loose, the clothes can be styled to look sexy, casual or sophisticated. They come in easy-care knit fabrics and cost less than $100, most less than $50. At Bloomingdale's, one young woman said before she disappeared into the dressing room, ''It's a little bit of money for a lot of look.''

Take, for example, the Multiples line, a 22-piece collection that includes tops, tunics, dresses, jump suits, skirts and pants. It's modular. One size fits all and the various pieces - basically rectangular shapes with arms or legs - can be mixed, matched, layered and interchanged to create multiple looks. Leggings and Tubes

It's also tubular. The accessories, which are key items, include 48-inch leggings and 24- and 48-inch cloth tubes, which can be worn as a bandeau, scarf, cowl, belt, wrap and on and on.

When you see women milling around the counters selling Multiples and Singles, a dress and jump suit line made by the same company, you might even think it all looks a little peculiar. At Bloomingdale's, Macy's, Dillard's or any of the more than 350 stores that carry the lines nationwide, all ages, shapes and sizes of women try on the same set of sample clothes, trading pieces as needed. The actual garments are sold in plastic bags or tubes.

Customers tend to look slightly bewildered at first. But the practical virtues of these generic garments soon become apparent, even as the urge to try bizarre combinations takes hold.

''I have a small child, and knits make a lot of sense,'' said Joan Corey of Manhattan, as she sampled a Singles jump suit with a tube, used as a sash, in Bloomingdale's. ''They're comfortable and don't wrinkle as much as other fabrics. They're simple, but the styles and colors are in fashion.''

Most of the pieces are 50 percent cotton and 50 percent polyester, machine washable, and for an additional option, they can be worn back-to-front. Colors are basic, usually red, white and black, with some seasonal variations.

At the Multiples counter at Macy's, Lillie Ward of Brooklyn was shopping for a trip to Hawaii. ''I'm looking for something that's going to be easy to handle,'' she said. She bought a jump suit with sleeves, $46; pegged pants, $27; a sleeveless jump suit, $39; a wedge-shaped top, $29, and two tubes at $9 each.

The concept of sizes is virtually eliminated. The Multiples line features micro-mini styles but can also accommodate a 60-inch hip. Popular among preadolescents, the clothes can be worn by pregnant women too.

It may all sound like the antithesis of fashion, but modular dressing reflects strong currents in contemporary clothing design.

''Multiples and I are like kissing cousins,'' said Donna Karan, the designer. ''The first fall collection I did was a tube skirt with a roll-down cuff waistband in black jersey. They've made a business, a brilliant business, out of a few wonderful pieces that work.''

The modular concept actually began as an exercise in anti-fashion, or at least anti-high fashion. Sandra Garratt, the designer of Multiples, who is generally credited with the mass-market version of the concept, came up with the idea in 1974 as a graduation project at the Fashion Institute of Design and Merchandising in Los Angeles. At the time, Ms. Garratt was making handmade, one-of-a-kind evening dresses. Her assignment, she recalled, was to ''design a line contrary to her baroque interest in couture dressing.''

''It was so far from anything I wanted to do,'' she said. ''This was problem-solving, as opposed to fantasy.''

It wasn't until five years later, however, after Ms. Garratt had worked in low-level jobs with the designers Mary McFadden, Halston and Zoran in New York, that a divorce and the financial pressures of supporting a child drove her back to her modular concept.