The Paris couture has survived three seasons of socialism in spite of prices for a custom-made dress that have risen to the equivalent cost of a small car. Each major biannual collection costs in the vicinity of half a million dollars to design and produce, not counting the furs, and the average profit for the house on each dress is only about 2.5 percent after social and labor charges, cost of the fabulous fabrics, and the saleswoman's commission.

The proverbial 5,000 international private clients who have been dressing in the haute couture since the 1940s have dwindled to fewer than 1,000, with only about 20 ''regulars'' from the United States. One has almost forgotten the postwar era when the late Barbara Hutton, the Woolworth heiress, used to purchase practically the entire Lanvin collection each season, merely checking off an occasional number on her program which she did not particularly want to buy. It was quite a twist on the usual procedure, where a client marks a few numbers on the list of a hundred or more models that might eventually interest her.

Yet the haute couture is the veritable engine of all French fashion, the country's eighth-largest industry. Per se, couture is obviously in the red, but the editorial publicity it receives twice a year from more than 600 specialized journalists and photographers who flock to Paris in late July and January is the contributing factor to all else: the ready-to-wear, men's wear, accessories, beauty products, and most of all, perfume.

All this seems as relevant to the average woman's life style as some science fiction movie, except for a few Arab princesses and an occasional millionairess who can afford to pay $3,200 for a shawl from Jean Louis Scherrer, a current price in his fall and winter collection. Yet the way that shawl is worn, a 10 -foot-long rectangle folded straight and tossed casually on one shoulder, appears the epitome of chic. your fashion portfolio.

Starting off with a basic color scheme, Paris adds flashes of brilliant tonalities that often turn into an equation of Stendhal's ''The Red and the Black.'' The comeback of the ''little black dress'' is like catching up with an old friend after years of silence. Solid black is set off by contrasts of fabric and texture or lightened with splashes of red, white, or other bright colors. Saint Laurent has employed literally hundreds of yards of black velvet. If you're looking for a new way to start the season, what better way than a dark velvet skirt as part of a purposely mismatched fall suit teaming to different jacket and tops already in your wardrobe.

There is no revolution in the couture, and designers have done nothing drastic to outmode the status quo. But if you are going to buy something new, you want it to be just that - new! And what is making a big difference this year is proportion. This is evident in the shorter cropped jackets and waist-length spencers, and in the wandering waistlines. The latter often drop down to the hips with widely banded blouson silhouettes, or are nipped with cummerbunds and draped sashes at the point nature intended.

Hem lengths are also important and one of the easiest ways to update. What a difference a couple of inches can make as you ponder the question in a full-length mirror: to go shorter, just grazing the knees, or to lengthen an old dress with a wide hemband in contrasting fabric - a trick everyone learned back in 1947 when Christian Dior's ''New Look'' changed the total image of fashion overnight.

What is brand new are the sleeve treatments, often echoing a retro feeling that runs through many collections. Shoulders are broad and sleeves nearly always mounted with tucks slightly above the natural line. There are updated versions of the old leg-of-mutton styles narrowing down on the forearm, or huge balloons the size of beach balls. These are usually featured in a contrasting fabric, such as two-toned iridescent taffeta setting off slim velvet sheaths. We have calculated that many of the giant puffed sleeves on Saint Laurent's ball gowns actually employ enough fabric to make another entire dress! One could easily achieve the same effect with a little bolero with big puffy sleeves to wear over some classic gown with a strapless decollete or narrow shoestring straps.

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