It's not what you wear but how you wear it that seems to distinguish clothes for next spring, judging by the collections of New York designers presented here over the last two weeks.

"Casual" appeared to be the latest buzz word - buttons left undone to reveal a lot of leg and chest, sleeves pushed up on jackets and coats as well as sweaters, necklines slipping off shoulders, hands shoved into pockets, hair purposely unkempt, pants and shirt sleeves rolled up, and loose clothes randomly belted and layered.

It adds up to a look that, when well planned, appears appropriate and relaxed, even sexy, much in the spirit of the new clothes from Paris.

When not, it looks plain messy.

"Working women in Washington, who have a penchant for a prim, Mary-Tyler-Moore-neat look for the office, already have started to buy and wear looser, more comfortable blouson tops and big tent shape dresses. They're likely to find variations of this new look much to their liking.

This casualness is not a dramatic change but the endorsement of a style that has been developing over the past few seasons. It represents the breakdown of harshly man-tailored styles (which sold extremely well in Washington), eased and softened to something far more feminine and sexy.

Yves Saint Laurent in Paris and Calvin Klein in New York, two designers who are master tailors, started the turnaround over a year ago when they began to take the stiff tailoring out of their clothes and moved toward something softer and more relaxed.

Klein replaced his man-tailored shirts with soft blouses, his slim or pleated skirts with easy drindls in supple fabrics.

Saint Laurent and others moved more directly to the opposite of strict tailoring with peasanty styles with all the gussying up the clothes could bear.

Now all of the excesses, the ruffles and trim, are gone. Clothes have been pared down to their simplest, most supple shapes. Jackets often have no linings, dresses are often without seams, loosetops, pants and tunics take their shape from the stance and movement of the wearer rather than a form injected into the garment by the designer.

Of course, this is not the only message from these two fashion centers, but it is a major theme with creators like Kenzo, Calvin Klein, Bill Kaiserman, who influence what many well-dressed men and women choose to wear.

(In fact, some leaders of the pack like John Anthony, Stephen Burrows, Karl Lagerfeld and Harriet Winter are already working on what is likely to be the next phase, a more parced down, controlled version of the current look.)

The new clothes will start to filter into Washington stores' stocks after the first of the year and begin to be well represented by March.

The ingredinets, which buyers on a three-week trek through showings in Milan, Paris, London and now New York have seen endorsed many times over, are these:

Oversized blazers, unlined, unfitted and worn over everything from bathing suits to evening dresses.

Oversized vests, an "essential" outer layer that lets shirts and tunics poke out from underneath.

Tent-shaped dresses or the big chemise, sensual for the wearer because of their lightweight material, and sexy to the beholder because of their see-through fabrics.

Pants of every length except the standard man-tailored variety, which has been replaced by a style tapered to the ankle. Shorts are part of assortment.

Fabrics (all natural) are so lightweight that even suede and wool become wearable for Washington's warm spring. Gauze in silk or cotton, fagootting that enhances the lightweight qualities and open weave leno treatments return as ways of adding to the bantamweight effects.

Evening clothes are often just as casual as the daytime styles, done in separates or in extended chemise or tent shapes. They raraly touch the floor. More often they're out off at the ankle as a far more versatile length, as well as a way to show off the new sandals.

Tiny patterns like Calvin Klein's rosebuds, John Anthony's flower clusters or Stephen Burrows doodles plus graphic plaids and stripes have started to reappear both for daytime and evening.

So where have all the T-shirts gone? They've been replaced largely by big summery sweaters in open and airy knits or big lightweight tops. T-Shirts show up occasionally as the bottom-most layer under multi layered garments.

Of all the New Yorkers, only Geoffrey Beene does an out-of-town run before bringing his clothes to Seventh Avenue. Aince last year he has shown (and sold) the identical collection in Milan before presenting it on the home front.

How's American fashion going over in Europe?

"Not bad," says Beene, whose businees this year is four times that of a year ago. And next week his Italian manufacturers will bring him a new contract to sign.

The first one was for one year.

The next is for 15.